I know that Python 2.5 is the officially supported Python version of web2py, but I've been coding all of my stuff with the assumption that this will eventually change to Python 2.6, or even 2.7. I have applications running on both 2.6 and 2.7 in production. But since I'm using Ubuntu 10.04 Server to run most of my sites, Python 2.6.5 is my primary target until the next LTS version of Ubuntu Server. Although I've been hearing that the new version of Mac OS X Lion will have Python 2.7.
I guess this all goes back to the fact that I think Python 3 is cool because it's new, and I'm sure I will start playing with it soon, but no distributions are really using or supporting it yet, so it probably will be another year or two before any serious Python 3 movement begins (not just with web2py). By then, there will probably some kind of huge speed boost, or, as you mention, an accumulation of small features that will entice more people to start switching to it. I know that it's been out for a while, but I still consider it to be somewhat beta since the barrier for entry is so high. I'm sure others feel the same way, but again, this will probably change soon.
