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.
I use mine as a headphone amp and wired an 8.2 ohm resistor across each speaker connector, since my headphones are 32 ohms, I thought it would be better for the amp to see a lower value. There is some loss of power, but the t-amp has plenty to drive headphones.