Stephen here, yes boothead is right. Using a combination of Haskell and Python you can make a really powerful trading system. Python has a lot of user-facing algorithm tools and Haskell has the robustness and parallelism for the backend that Python doesn't.

If you're interested in advice on how to bridge the two worlds let me know, there's a lot of of upcoming technology ( LLVM, Blaze, pipes, zeromq, cloud-haskell ) that could be very useful.