I've been learning Common Lisp, and I've been slowly pick at the Bittorrent spec over the weekend to try to get something going.
My only experience so far with network stuff has been toy programs in Erlang, which makes TCP/IP communication fairly painless. I was worried about how that experience would map over to Lisp when I found:
It describes itself as an Erlang-inspired multiprocessing model for Common Lisp! Weee!
(And by picking away at the spec, I mean I have a bencode library written. That's the easy part, I think.)