I'd write for 2.4, even though 2.5 should be coming out "shortly". There aren't many significant changes to the whole language between 2.4 and 2.5. Probably the best thing is write for 2.4 and have a sidenote stating where 2.5 operates differently.
The Python 3 timeline is almost a moving target right now; personally, I don't think it will be out before next winter. Maybe a beta but I doubt the full version. -- http://mail.python.org/mailman/listinfo/python-list
