John Cressler's research group at Georgia Tech and IBM have created an SiGe chip that runs at 500GHz. They achieved this by cooling the device to 4.5K. But it wasn't a slow technology to start with, having a room-temperature switching speed of 350GHz.
If I did this right, Landauer's Limit says that at 350GHz of bit destructions @300K, each transistor would generate 1.5nanowatts. So, a billion-transistor chip would have a physical, fundamental limit of at least 1.5 watts, unless reversible computing is used. Still, we're clearly orders of magnitude from that limit... (I may have dropped a small constant there, but I think I'm within a factor of two.)