According to the dev, the Wii U's CPU speeds run at 1.24 GHz per core with 3x PowerPC type cores. Additionally, the GPU speed of the Wii U is 550 MHz.
The current generation Xbox and PS3 consoles run at 3.2GHz, but the dev notes that clock speeds aren't everything: "So yes, the Wii U CPU is nothing to write home about, but don't compare it clock per clock with a 360 and claim it's much worse. It isn't."
@digitalfoundry 1.243125GHz, exactly. 3 PowerPC 750 type cores (similar to Wii's Broadway, but more cache). GPU core at 549.999755MHz.-- Hector Martin (@marcan42) November 29, 2012