Someone built a web server in Erlang. It scales really well on multiprocessor architectures because of its threading model. I believe that Haskell has a similar threading model and therefore could achieve similar performance.
Via John Udell, Patrick Logan makes this point in the contect of pointing to this register article is saying that multi-core computing is the future and this bodes well for the sorts of threading models found in Erlang and Haskell:
A single Erlang node on a single CPU today can comfortably get into the tens of thousands of dynamic processes. What would your system look like running hundreds of thousands or a million dynamic processes and lot of activity is spent collaborating with other systems also running at that scale?