Only administrators can see your IP address. If theymos receives a request from the government or whoever then he'll likely have to give up public information depending on the details of the case. Though, how likely would it be that the government would request theymos to release information. Especially if you are just browsing and taking part in discussion.
theymos has in the past gave up public information:
https://bitcointalk.org/index.php?topic=881488.0Let's just say that case was quite substantial.
I am willing to cooperate with police on real scams. Whenever someone asks me to release a scammer's IP, I tell them to have police email me from an official police address. I have received police requests a handful of times. Mostly the cases were real scams and I gave the police the requested info. In some cases I've rejected their requests. For example, I refused to give information to some foreign version of the SEC because securities laws are unjust. Of course, you should not trust that I will act in your best interest. If you want to be anonymous, then you must use Tor (or whatever).
Here's some information too, outdated but information nonetheless:
https://bitcointalk.org/index.php?topic=38253.0