You can wake your Pi by briefly connecting pins 5 and 6 on P1. This is handled by the firmware. It will cause the kernel to be reloaded from scratch though, so is pretty much the same as a cold start.
You can also connect the two pins of (the not fitted by default) P6, which is (I believe) a hardware reset.
I'm not aware of any other way to wake a Pi from that state, other than killing the power.