(Haven't noticed an appreciable difference between 3.0 and 3.5...)
This is rather strange, because I don't often think about averages because I am usually looking to introduce extremes, and test the limits of material. I'll stick with discussing face to face sessions since I think there is a huge difference between that and online play. I usually go into a session with plans to accomplish a certain number of encounters. This is usually planned as a full adventure, with some of the "side" encounters trimmed out, so we wind up with the meat of the module (the same way I would run something for a tournament or gameday). Often I am running some material as a playtest, I usually have four players (with one character each), and we want to try and maximize the time we have. That said...
My guys ROCK! (...not that others don't, mind you...)
We usually play from 5 to 7 hours (tending to the long side) in a session and manage to knock off between 4 to 7 encounters, IIRC (averaging 5). Most of those encounters last from 5 to 12 rounds (averaging 8, perhaps less, I guess). We playtest material from 1st to 12th-level (sometimes 6 players of 10th-level taking on 12th-level encounters, roughly). Some of the combat encounters are cleverly avoided by the group, so perhaps the average rounds per encounter should be considered lower and the average number of encounters per session should be considered a bit higher. Since I am trying to get a real gage of how deadly things are, I do not fudge and prefer to either keep some extra characters handy or allow a bit more healing to be available (but that happens between encounters, mostly.)
Oddly, I would consider my gang a hardcore
ROLE-playing bunch and we have tons of fun (and spend lots of time) interacting, so I am not sure how we manage to get so much accomplished. We play at the Games Plus gamestore, take a food break at a good stopping point, and rarely have anything that distracts us from our games.
My gang would probably also claim I tend to crack the whip a bit as we play, to keep things moving at a brisk pace, but that's another story...since it is them who really wind up doing the moving along...
(Higher level playtesting I often do online as it involves more number crunching and checking for hidden broken bits that can nulify an encounter's effectiveness and playability.)