Sounds like my situation too. We had DS (he'll be 3 in Sept) in underwear full time for more than 2 months(except at bedtime), but his chronic poo accidents started to lead into chronic pee accidents (at first he would pee 95% in the toilet), and I didn't feel like it was fair for his daycare provider to deal with that, so back into pull ups he went. DS is very smart (like 'gifted' smart), so I continue to be amazed that he is potty training later than his 'not particularly intellectual' older brother (who's now 6). He completely seems to understand what he is doing, is fully capable of going in the toilet (standing, sitting, big potty, little potty - you name it, he knows how to do it), but has chosen not to do it 'correctly'. We even have gentle conversations about it - one where he even said 'yes, he likes to make mommy mad/sad'. :( He will poo/pee almost exclusively in the toilet for our former nanny, so I'm actually hiring her back full time this Sept. (obviously, not something everyone can afford).
At any rate, I have also tried everything that everyone has mentioned in their posts (toys, candy, cheering, yelling - mostly DH, even a spanking or two, letting him sit in wet/poopy diapers), with the exception of the Baby Whisperer, and the 'making them clean up their own mess'. He woke up just now, and I made him remove his pee filled pull up, and handed him some wipes to 'clean up'.
I guess I'm not really offering up much advice, except I feel your pain. I'm going to give the BW suggestion a try, since I think DS might be a good candidate. Good luck.