By way of update from the standards trenches: threads are indeed a part of the WebAssembly plan, and we're also actively working on standardizing the SharedArrayBuffer API for JavaScript, which would allow an asm.js implementation with threads (either as a direct compilation target or as a WebAssembly polyfill). Shu-yu Guo has been working on drafting the SharedArrayBuffer spec and is planning to present an update in the next standards meeting in a couple weeks.