The Media Center remotes will wake/sleep your system. Windows Media Center does a fantastic job with sleep, actually. It'll wake up to record, then go back to sleep. If you hit "sleep" while it's recording, it will shut off the video output, then sleep when it's done.
You can generally wake up most PCs via a USB human interface device (HID), and there certainly are IR receivers that act as HIDs. Not that I've tried it.
