Darkstar: ? (Says it's about behaviour not speed. Nevertheless I'm optimistic because of their in memory data store and - fingers crossed - seamless horizontal scaling).
Erlang OTP: Nuff said. Have to write the whole thing from scratch, but I can't think of a better friend along the way than Erlang.
Ejabberd: Mebbe. Seems to me that the messages are too big - we tend to rock somewhere between 30 and 200kb.
C# raw sockets: Benchmarking soon, only just finished it. Took longer than I expected, there was plenty of shittiness, and I'm going to have to experiment with some assembler jockey style way of flushing a byte[] instead of reallocating a new one.
Ready and set and go now!