Restart or shut down your computer and when it starts to boot up, hold down the 6 and 4 keys. This will boot Snow Leopard into 64-bit mode. [ Enter to win a copy of My New Mac, Snow Leopard Edition ] ...

Ok, so I'm at a client's house, and their daughter's new imac's keyboard no longer acknowledges the spacebar. You'd think it would be a hardware issue, but a different keyboard has the same problem.