You could harvest IP addresses from the IRC channel, but you could also do that by running the client and noting down which peers you connect to. Unless you're running behind Tor or I2P, it's not hard to figure out which IP addresses have connected.

However, assuming the government had this list, what would it do? It's hard to bring charges against a large number of individuals scattered throughout different jurisdictions, and historically, the US government hasn't gone after the people using alternative currencies; they'd go after the source.

They could shut down the Bitcoin site - which would be quickly moved to an international domain. They could try and prosecute the lead programmers of the Bitcoin software, but this would not be particularly successful. It wouldn't stop Bitcoin, and I think it would be hard for them to show the currency as being illegal without, for instance, affecting MMO currencies as well.

I figure their best move would be to shut down the currency exchanges, or at least tie them up in court long enough to damage confidence in the Bitcoin economy. However, if the exchanges win, then Bitcoin has an official seal of legality, and a huge heaping of press attention.