Hmm, a bit intriguing. My first thought is that you could set up a bounced mail box that PHP could read, parse it for addy's, and go from there. How would you call it, though? Maybe as a cron job, via lynx, wget......<?> And what about its permissions and location? Hmm, maybe someone else will grab and go with this one...my first thoughts are usually not quite right.
Also, you should probably look around a bit to make sure you're not reinventing the wheel.....
