It's not. It might be possible to implement it in the future with Web Workers, but it'd be a stripped down implementation.

You could certainly run the managed bits of System.Threading through the translator, though; I've done that before. But the shiny bits happen to live on the native side of things, in both MS's CLR and Mono... :(

Gotcha, looking at the JS. Still very impressive. Been interested in the Script# project for awhile, this looks along same lines. Good stuff.