My dd is 3.6, and has intermittent episodes of soiled pants - not a full scale poo though, just a spooful (sorry, tmi). Sometimes runny, sometimes not.
She doesn't seem to be constipated though, which I understand is the usual cause of dirty pants. Also, the problem seems to coincide with dairy consumption - dd was bf for two years, and has always hated cow's milk though she will go through phases of loving yoghourt or cheese for a few days. As I said above, these seem to correlate to the soiled knicker incidents, and I am wondering if anyone else has come across this?