The IBM Power6, which they claimed would scale to 5GHz and beyond, is going to be launched next month. While not at quite 5GHz, they are certainly close, with the first iteration running at 4.7GHz. That puts it at roughly twice the clockspeed of the fastest Power5. Coupled with up to 8MB cache, it is aimed at very high-end servers. People other than IBM seem to be optimistic about its potential, too:

"Go back a few years, and the Power brand was an also-ran in the big iron chip race _ in fact, it wasn't really clear how committed IBM was to its own chip development," said Gordon Haff, principal IT adviser for Illuminata Inc. "But IBM decided to double down its bets on Power, and the results have been pretty impressive."
Despite the extreme speed of this CPU, it falls under IBM's energy efficiency initiative they announced earlier in May, making it an interesting launch indeed.