<p>Does anybody know how I can see the actual machine code that <a href="http://code.google.com/p/v8/" rel="noreferrer">v8</a> generates from Javascript? I've gotten as far as <code>Script::Compile()</code> in <code>src/api.cc</code> but I can't figure out where to go from there.</p>

You're on the right track, I think.
It looks like you need to get from Script::Compile to Compiler::Compile, which will lead you to the code generators (codegen*.cc and .h).
All of this to say that, looking at codegen-ia32.cc, if you define ENABLE_DISASSEMBLER
when you build, your disassembly should get printed, I think.
Of course, all of this is just from a quick browse of an old copy of the source I have here, so YMMV, but I think this should work.
(Looking at your post again, I see you're looking for the machine language, not the assembler -- I'm not sure, but you might have to modify the logic if you want the assembled code output rather than its disassembly)