You can use Google's new JavaScript minifier, Closure, here.

Besides whitepace compression, it shortens the names of local variables, and, in the Advanced mode, can remove unused code.

I think it has some built-in knowledge of the Closure Library too, so if you use that you can pull in only the parts that are used?

UPDATE - after playing with this for a few minutes, it's clear that Closure is WAY MORE than a minifier. Even in the Simple mode it can do unused code elimination, code folding, common subexpression elimination, unwinding function calls, etc.!