With their recent announcement with Facebook to make the Unity web player plugin experience more pleasant, this is much less bad news.
And hopefully they're working on an HTML5 target as well.
So, just another nail in the coffin for Flash at this point.
I'm pretty happy with them focusing on making the web player install easier / more seamless. That will achieve the same goal and be better than running the game in yet another platform that can have its own bugs.
According to Eich's talk Friday, ASM.JS is around 2x slow as native, which is acceptable, but should improve. Before that other WebGL solutions were 10x+ native.
I know Tojiro has already had Unity working unofficially on WebGL for a couple of years.
So I have a pretty good estimation of what's coming...