JChung2006 said:

I'm surprised that you didn't mention GWT or Script#.  GWT is more popular than Volta, Script#, and JSC combined.

It isn't too hard to envision an optimizing JavaScript JIT compiler being able to optimize computationally intensive code with no I/O or DOM manipulation to C-level performance figures. These optimizations are largely (but not completely) irrelevant to Web programming. I don't really see people flocking to reimplement their performance-critical code in JavaScript. They should be focusing on speeding up the DOM and browser rendering stacks instead.

I'm surprised that you didn't mention GWT or Script#
I completely forgot about Script#, thanks!

I'm personally coming from the perspective of bringing traditional .NET desktop development to the web browser, so Volta and JSC seemed the relevant compilers/engines (I could remember) that would generate JavaScript from C# or VB. But I see your point that TraceMonkey isn't the first to JIT/cache to accelerate web apps (Bass mentioned this earlier, too).
These optimizations are largely (but not completely) irrelevant to Web programming
That's basically what I was wondering (I don't have any experience with JavaScript).