The Everquest D20 player's handbook has infravision that functions basically like darkvision (i.e. limited range), but through seeing heat, and it says in that book that since the playable races with infravision are warm-blooded, they are effectively lit up like torches to infravision. I don't know if it's OGL or not so I won't tell any specifics, and in any case I don't know if it's realistic.
Here's something I've got typed up for a race in my homebrew setting. They're essentially lizardfolk that are more human-like in appearance than the standard lizardfolk, less primitive/burly/etc. and more civilized, charismatic, and talented in magic and psionics. They are of the Humanoid (Reptilian) subtype and are cold-blooded like most reptiles (I've also typed up a description for game-effects of cold-bloodedness for these Medium-sized humanoids; the effects would be less drastic for a smaller reptile or more drastic for a larger one). Here's my description for their Heat Sense ability, which isn't infravision but similar enough. Note that this is linked to their eyesight partially, whereas pit vipers in nature have an entirely separate set of heat-sensing organs, and infravision in D&D may or may not be sight-linked. Supposedly, a warm-blooded creature with infravision or heat sense might not be able to see anything with those abilities because their own body heat would create a heat haze about themselves, as S.K Reynolds points out. Note that my kaid race's Heat Sense is a crude, weak imitation of infravision, which means basically that it's relatively balanced for PC use.
Heat Sense (Ex): A kaid has special sensory organs akin to the heat-sensing organs of certain viper species, though much more crude, allowing the kaid to detect various heat sources and their general form. Heat Sense is an extraordinary ability, that functions within the kaid's line of sight but only out to a range of 60 feet. As this is partially based on their eyes and vision, a kaid's Heat Sense is blinded whenever their sight is blinded (not just when effectively blinded by darkness or the like, however). Kaid have a heat-sensing organ behind their nostrils, another underneath their tongue, and one below each eye, so as long as the kaid has at least two of these functional, their Heat Sense is also functional. Kaiden usually keep this extra sense inactive, but may spend a free action any number of times on their turn to activate or deactivate their personal Heat Sense. As long as Heat Sense is active, the kaid suffers a chance of being blinded and stunned briefly if they observe a very intense heat source. Any creature of at least Gargantuan size with the Fire subtype will trigger this penalty if seen through Heat Sense, as will any source of heat that is seen through Heat Sense if it could deal 8 or more dice (or 30+ points) of fire damage in 1 round or less, or if it would normally deal 6 or more fire damage dice (or 22+ points) every round. Whenever this weakness is incurred, the kaid must attempt a Reflex save against DC 20, in an attempt to reflexively shut down their Heat Sense, and a failed save results in the kaid being blinded for 1d3+1 rounds and stunned for the same duration.
While this ability is active, the kaid senses any corporeal living creatures within its range as long as those creatures have Dexterity and Constitution scores of 4 or higher each, since creatures with lower scores do not generate sufficient body heat to be detected by the kaid's Heat Sense. Creatures of Diminutive size or smaller cannot be detected by the crude heat-sensing of a kaid. Creatures of the Construct, Dragon, Ooze, Plant, Undead, and Vermin types cannot be detected this way unless the GM believes they are generating very considerable amounts of heat, which most creatures of those types do not. Creatures of the Aquatic, Cold, Incorporeal, Reptilian, or Water subtypes likewise cannot normally be detected by a kaid's Heat Sense. Kaiden thusly cannot usually detect themselves or others of their race through Heat Sense. However, creatures of the Fire subtype are always detected regardless of their other size, status, types, or other subtypes. Objects and energies within range are also detected if they put out heat equivalent to or greater than a torch or lantern. Heat sources like torches or greater will effectively illuminate anything within 5 feet of them for purposes of detection through Heat Sense. Heat Sense is blocked by one foot of soil, three inches of stone, one inch of metal, or a thin sheet of lead, but only if such obstructions provide 50% or greater cover.
With Heat Sense active, the kaid receives a +4 circumstance bonus on checks to find or follow the tracks of creatures that could normally be detected by the kaid's Heat Sense, but this bonus is reduced by 1 for every 20 minutes old the tracks are; a kaid can thus tell generally how old the tracks are if they are less than 80 minutes old. A kaid's Heat Sense allows them to automatically detect the presence, location, size, and general shape of any creature in range whom the Heat Sense is able to detect (as per the above guidelines). Heat Sense does not detect these details with regards to invisible creatures, since the Heat Sense is dependent on detecting infrared light. It can detect the tracks of such creatures though, as long as they would otherwise be detectable by Heat Sense (that is, aside from being invisible), since invisibility does not prevent them from leaving tracks; in this case, by following the tracks, a kaiden may be able to tell the general location of a creature, though the creature would still mostly benefit from concealment. Heat Sense does not function in nor through magical or psionic darkness, since such unnatural darkness likewise blocks infrared light, even that emanating from tracks. Heat Sense does not reveal the exact features of a creature to the kaid, only the creature's general shape, size, and specific location. This is sufficient, however, for the kaid to attempt Sneak Attacks, critical hits, or the like, since they can discern enough about a creature to make a good approximation of its vulnerable points (if any, and no more easily than they could with a normally-visible creature). In fact, the vital parts of a creature often generate more heat than the rest since they tend to be more active, thus facilitating the capacity to make critical hits and the like normally against creatures revealed by Heat Sense (if they are subject to such attacks).
Without the complexities and open-endedness and possible abuses of being able to see heat. 
; just don't ask for explanations 
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