If you're looking for a good library/framework, then Twisted is a really good choice. It's a piece of battle-hardened software and used by several big vendors (like Facebook, Rackspace .etc).
Mmmm.... Twisted....
I started defining a BitcoinProtocol class derived from twisted.internet.protocol.Protocol for my cross-implementation at-the-network-level testing project. I plan on using Twisted and Trial (the Twisted unit testing framework) to feed canned block-chains to "empty" nodes and make sure they Do The Right Thing (reject blocks that violate the blockchain rules or contain invalid transactions, accept blocks that contain weird-but-valid transactions, etc).
https://github.com/gavinandresen/Bitcoin-protocol-test-harness/blob/master/BitcoinClient.py
Anyway, the BitcoinProtocol class might be a good place to start for anybody who wants to do some python-based bitcoin network programming. Good example to demonstrate is a little dump-blocks tool I wrote to spit out a blockchain in JSON format:
https://github.com/gavinandresen/Bitcoin-protocol-test-harness/blob/master/dumpblocks.py
(I'm a Twisted newbie, so improvements, suggestions, etc are very much appreciated)