I see from comments on Robert Scoble's blog that Downhill Battle have been asked to remove the SP2 torrent links. The reason is apparently that they don't want things to be downloaded from other sites. I suppose it prevents MS monitoring how many people have actually downloaded it - but how can they guess how many times a particular download, particularly of the network install, has been used?
BitTorrent has mechanisms to check that the download was received correctly: the Torrent file includes precomputed hashes for each block so that the client can determine whether it has received that block successfully. These hashes use the SHA1 algorithm. The whole download is also hashed - a download can be made up of multiple files. This hash is used to locate the peers.
In the particular case of SP2, the file is also digitally signed by Microsoft. This ensures that it really is the genuine release, or at least that the file hasn't been modified since it was signed and it was signed by an organisation claiming to be Microsoft. It appears to have had the right effects, so I'm sticking with it.
Interestingly, though, Microsoft haven't tried to take down the tracker. Clearly there's a lack of understanding in how BitTorrent really works.
I'm trying to decide whether to host the .torrent file myself. I'm still hosting the torrent itself - upload rate currently 24KB/s.