Kinda redundant, as we're guaranteed to get a processor upgrade next generation (it's already been mentioned several times, anyway). Underclocking already comes with OS2008, it just scales to user demand. A selector for this would be rather pointless, you either get more cycles with a slight reduction of battery life to get something done quickly or fewer cycles with less of an impact on battery life that takes longer to complete—the result is mostly the same.

Plus, these sort of things should be transparent to the user, anyway.