On Leopard you can enable it by defining the environment variable:
HOMEBREW_USE_LLVM
I didn't enable it on Leopard by default because it doesn't get as much testing, and I don't want to rock the boat. On Snow Leopard people are forigiving of issues because it is new and many compiles fail anyway. And if you installed Snow Leopard you *want* the cutting edge.
ClosesHomebrew/homebrew#29