I know a few years ago there was a whole bunch of leaked emails from some company involved in work for the 'big brothers' published on some site (it isn't there any more). Basically, they have had a patch added to many trackers to send them a list of who downloaded what. I don't know if it was done with or without the cooperation of the owners of the trackers. So they always know what you are doing - no privacy there. Now, mostly they don't have time to go after downloaders of old stuff, but they certainly will be after ppl who grab stuff that still makes them a lot of money. You're lucky you only got a letter - often the first you'll hear of it is a court summons or nasty demands for $k.