With a healthy amount of coding, I've gotten very comfortable with python, although I still feel like I'm not quite utilizing it idiomatically. As such, I've been going through Programming Python ( http://oreilly.com/catalog/9780596009250/ ) - which is very well written, IMO - as well as reading a healthy amount of other people's source code, notably CherryPy ( http://cherrypy.org/ ), since I use it a lot.
If you're experienced with other languages, I'd suggest implementing something like a tetris clone as an exercise in learning the language. If you're only experienced with web-dev (as is often the case these days), I'd recommend implementing a few small web-apps with CherryPy - it's the most "pythonic" "web framework" I've seen so far.
Oh, and keeping an eye on the mailing lists ( http://mail.python.org/mailman/listinfo ), particularly python-list, python-ideas, python-dev, and python-3000, can be very enlightening.