Turning a PC on using Insteon and the ISY may be useful if you are using a PC as a media hub. Imagine setting up a Keypadlinc button as a macro for turning on a PC, which boots without a login screen and runs your favorite media application (e.g. iTunes and Airtunes, in my case).
This is probably straightforward to implement. In looking at the way the switch on an ATX computer case works, it looks like an Insteon-compatible I/O controller like EZIO2X4 or the IOLinc can be used as a means to close the momentary contact circuit signalling the PC power supply to turn itself on.
The ISY program mapped to the Keypadlinc button could also used to turn on an audio component and set it up in the correct mode for the playback of audio from the computer.