Alchemy is a research project that allows compilation of C and C++ libraries into code that runs on the ActionScript virtual machine used by Flash. Ralph originally tweeted:
Had a quickshot porting #WebM codec to Flash 10 using libvpx and alchemy. Surprisingly, got a preliminary version to work.
This is good stuff and it'll be worth following his twitter feed if you're interested in a pure flash implementation of a WebM decoder.