The IP address is tied to a specific computer that only I have access to (password protected). I spend a lot of time on this computer, but I can't think of anything I've done that would be considered illegal. There have been occasional website visits late at night to watch dirty videos and read dirty stories, and while the stories may involve illegal activities, they are other people's stories, not mine. These late night ventures would warrant embarrassment at most, certainly not a subpoena.

When my ISP says "law enforcement", does that necessarily mean that a specific law enforcement officer is requesting it? Or is it that every subpoena must be backed by "law enforcement", but could be initiated by a private party over a civil suit or private investigation? Is there any way I can get my ISP to cough up a copy of the subpoena? Or would that even help?

As you stated, if I've done nothing, then I should have little cause for alarm. Perhaps I should just let it go and wait and see if anything happens. I've never had my information requested by subpoena (and I've been on the internet since it's conception). My initial cause for alarm is based mainly on my ignorance of subpoenas, and fear that only something really bad would trigger a subpoena.