Using bittorrent to download the block chain is a really interesting idea. I wonder if it'd go faster that way because of all the potential peers to download from... interesting idea.
as mentioned above, it's not the number of peers that makes it go faster, it's that it grabs any available data, in any order, and reconstructs all those pieces in the right order once they've been downloaded
the bitcoin client on the other hand, downloads block 1, 2, 3, etc, in that order... if the peer that is sending them block #1000 is slow, that means all blocks #1000+ are delayed