I have a USB-UIRT inside my HTPC configured to wake my PC from sleep (S3). It works great! S3 sleep is indistinguishable from power-off, other than the ~5W power consumption while in this state. Of course, if the power is interrupted while in this state, you'll have to hit the power button next time you want to use the PC.
There exist devices that power-on a PC, but AFAIK most HTPC builders use S3 sleep.