Python 3.0 Is Out
I posted a short article a while ago about 3.0 (in)compatibility, but the differences between 2.6 and 3.0 aren't so great. It's perfectly possible to write 3.0 code that will run on 2.6 too, as most of the language hasn't changed at all.
The preferred strategy for writing code that runs on both versions is to write in 2.6 and then apply the 2to3 converter and verify that it produces a correct 3.0 program. There's no guarantee that it will, so you may need to paraphrase the 2.6 code a few times before you get a transatable program.
Once all the third-party modules you and extensions you rely on are 3.0-ready, and you no longer have clients requiring 2.6 version of your software, you can simply drop the 2.6 compatibility requirement and start to make use of the few 3.0-only constructs that have been introduced.