DMG, p195
Hide in Plain Sight (Su): a shadowdancer can use the Hide skill even while being observed. As long as she is withing 10 feet of some sort of shadow, a shadowdancer can hide herself from view in the open without anything to actually hide behind. She cannot, however, hide in her own shadow.
PHB, p48.
Camoflage (Ex): A ranger of 13th level or higher can use the Hide skill in any sort of natural terrain even if the terrain doesn't grant cover or concealment.
Hide in Plain Sight (Ex): While in any sort of natural terrain, a ranger of 17th level or higher can use the Hide skill even while being observed.
To use the hide skill you must normally have cover or concealment of some kind, and cannot be directly observed. SD HiPS: As long as you are within 10' of some sort of shadow, you can hide without cover or concealment, even while being directly observed. Period. End of statement. It doesn't say "you can hide without cover or concealment, even while being directly observed as long as the person observing you doens't have darkvision." Just without cover or concealment, and/or while being observed. Period. The combination of a ranger's Camoflage and Hide in Plain Sight abilities recreate this as well, but trading the requirement for 'within 10' of some sort of shadow' for 'must be in natural terrain'. (plus it's an Ex, which is rather cool).
The light rules and darkvision rules have no bearing on these abilities. None. Zero. The only things that HiPS does it remove restrictions in which the hide skill can be used. Now for the blow-by-blow:
And since darkvision is not usually effected by shadows shouldn't the HiPS rule specifically mention that it has this added bonus feature for us to assume that it does?
Nope, no need to. Darkvision lets you see thru darkness, and thereby observe the SD/Ranger. But because if HiPS they can hide while being observed, so it doesn't matter.
Would something that uses the hide skill and doesn't mention magical invisibility somehow be granted that power?
Nope. Apples and Oranges. The
Invisibility spell grants you a bonus to your hide skill (+20 if you're moving, +40 if you're not). Hide in Plain Sight does not. Any schlub can be unseen if made magically invisible. With HiPS you have to be a skilled hider on your own merrit or people will still see you. A 17th level ranger with 0 ranks in Hide can attempt to HiPS because of his abilities, but the chances are almost certain that anyone looking for him will spot him without difficulty.
Perhaps the shadows are magically made into solid cover, or a concealment that darkvision can't penetrate. Well in each of these cases, what does the darkvision character see then? Does HiPS somehow make the shadowdancer invisible and thus the darkvision character just sees a whole hallway? Or does he see a normal hallway and then one black spot of shadows he just can't see through? I don't require total realism but this is a stretch. The dwarf scratchs his head and says to himself: "I can see the whole room except that strange shadow to the side of me. I wonder where that crazy rogue could've gone? I guess I'll go get some Ale at the --- AAHHHGGG!"
The dwarf in your example would see a blank hallway. No 'mysterious' impenetrable dark spots, nothing. Just the hallway, minus the hiding person. Don't forget that a HiPSing person still makes noise if they move (a seperate Move Silently check), still give off scent, still vibrate the ground, you name it. The dwarf can guess where he thinks the person hiding may be, the exact same way he can with any other opponent he can't see.
Could a person with see invisibility find the shadowdancer?
That's a grey spot. HiPS
is a Su ability for SD, but the See Invis spell description says "this spell does not reveal creatures who are simply hiding, concealed, or otherwise hard to see." I'd say rule this one as you see fit.
If not then you are saying that its neither a magical invisibility or a mundane hide skill. That would require a new state that is neither hiding or invisible.
It doesn't require a new state; it is the use of the hide skill (remove the appelation 'mundane', it's pointless) that has had some of the normal restrictions on what conditions must exist for its use removed via certain class abilities. Hide in Plain Sight is still the use of the Hide skill.
This is nonsensical and since HiPS isn't clearly granted the power to escape darkvision I just wouldn't allow it.
*shrug* Rule 0: the DM is always right, even when he's wrong. It's your game, man. If you want everyone with darkvision to negate a shadowdancer's primary schtick, or a ranger's 13th and 17th level abilities, you're free to do so. Don't expext many shadowdancers or high level rangers in your games, though.