What jurisdiction is the forum under
The forum is under US jurisdiction and it is indeed subjected to US subpoenas. This information can be viewed in the
Privacypage of the forum.
under what circumstances could you be forced into providing information relating to a forum user?
When theymos receives a US subpoena, I don't think it is possible to ignore it completely as the forum works under the US laws. But, theymos on instances such as the BFL case has tried to limit the private information as much as possible. If on other scenarios such as other country police contacts theymos for certain private information on the forum user (who have been found guilty) and if he is aware of the issue he might help the police with investigation.
Meanwhile in the BFL subpoena, the government demanded the PMs of all the users who mentioned BFL but this has been lowered down.
See this :
I had to release all database info on a few employees/ex-employees of BFL (including their PMs), plus a complete copy of every thread in which anyone mentioned BFL or in which a BFL employee participated. (It was a huge hassle to put all of this info together.) The subpoena originally demanded all PMs that even mentioned BFL, which is ridiculous, but I managed to get this part eliminated.