There are a whole lot of possibilities for a "webapp stack" in python. What are you running your sites on top of?
My point is not that it's impossible to run 2.6; but setting it up requires a level of sophistication with Python/*nix that I can't put on the shoulders of my organization, yet.
If I were working with a couple of smart hackers, this would be a non-issue. But my feeling is my organization is like a lot of others, where anything that's not a tagged, stable "sudo apt-get install" away is not going to fly.
I'm a lone guy, managing my own servers, with complete control over their environments. That's quite a bit different than a business environment, corporate or not.
http://code.google.com/appengine/docs/whatisgoogleappengine....
2.5.2 is "latest stable" for me, I've had no problems with it.
(I think web.py 0.3 and possibly the trunk of Django can handle Py2.6, but obviously they're not going to drop support for older versions any time soon.)
Just for kicks, try getting a stable webapp stack running on 2.6. I'm still not near ready to put that into production (and I run tiny, tiny websites).