Yes data is shared with the US government when subpoena'd. If it is worth fighting the subpoena, such as if there is a technical issue with it, I'm sure theymos would fight it.

One example was the account the FBI alleges belonged to Ross Ulbricht, the creator of silk road, was subpoena'd and theymos had to comply. Though I believe all that was produced in that case were publicly available posts along with deleted posts.

Another would be the ButterflyLabs incident. IIRC the government initially requested all private messages on the forum that contained the phrases BFL or ButterflyLabs, along with all private messages to/from certain accounts, most of which were BFL staff. Theymos got this lowered to just private messages to/from certain accounts and complied.

In both of these cases theymos notified everyone who's data was handed over.

Theymos also stated that he will in certain cases hand over information such as IP addresses of scammers to foreign law enforcement even when not legally compelled to when it's obvious the person was a scammer.