I'd rule out Erlang, if they wanted a functional language they'd probably go with F#, seeing how this is MS.

I don't think this is enough info, even for an educated guess. What they're describing (single threaded/callbacks) could even be C/libevent, no?

Edit: Also,

> Every microsecond of CPU time is accounted for and justified!

...for me, this rules out any kind of VM contained language