Has anybody modded the Sonic Impact into a portable headphone amp?

Yes.. Quite a few folks over at head-fi.com have done it. The mod usually consists of adding a resistor across the outputs to match the impeadence of the headphones to be closer to 6 ohms (the load a stock SI is optimized for).

There's been a lot of discussion regarding the ultrasonic switching noise produced by the SI, and whether or not it's good to couple that to your head with headphone, though.