Is the forum willing to disclose a list of IP's used by a user when requested by a Law Enforcement agency?
Yes.
If so, is it a blanket thing where all requests are complied with, or is each request checked by the forum staff before giving out the information?
Theymos (the only person who is supposed to handle these requests) will only release IP addresses to law enforcement if he personally believes that a legitimate crime was committed (regardless of the laws of any country) and he personally has access to strong evidence that this was the case. For example, if the German police contacted him to request the IP addresses of a Holocaust denier, he would reject the request outright.
Has any such requests been received yet?
Yes, all the time. However, since it's hard for law enforcement to figure out how to contact Theymos, he generally only gets requests that meet his criteria.
We will also release IP addresses to the general public if certain criteria are met:
https://bitcointalk.org/index.php?topic=38253.0