We think about whether we should convert a quite large python web application to Python 3 in the near future.
All experiences, possible challenges or guidelines are highly appreciated.
My suggestion is that you stick with Python 2.6+, but simply add the -3 flag to warn you about incompatibilities with Python 3.0. Then you can make sure your Python 2.6 can be easily upgraded to Python 3.0 via 2to3, without actually making that jump quite yet. I would suggest you hold back at the moment, because you may at some point want to use a library and find out that it is only available for 2.6 and not 3.0; if you make sure to cleanup things flagged by -3, then you will be easily able to make the jump, but you will also be able to take advantage of the code that is only available for 2.6+ and which is not yet ready for 3.0.
Make the Move to Python 3 - Best practices