My son is about the same age as yours, also 3 in May 09. I was training him inconsistently too, which was probably not helping. I bought him underwear last week, but mostly, I've been letting him go bottomless while we're home. He'll start to "dribble" a bit then make a shocked noise, and I say "go to the potty!" and he runs right in there, pees, looks at it and gets all excited. Sure we have lots of accidents, but we're having lots more successes than ever. Yesterday he even asked to go to the potty himself as we were coming in from outside, got to the potty, and went in the potty! I never would have believed this could work, but I've heard of lots of people doing it so I said, "why not?"
We still use a pull-up for sleeping because I'm not cleaning sheets everyday. One step at a time. Good luck. Oh and I started using his favorite TV show as an incentive, too. Curious George--he watched it all the time, but now when he asks I say you have to use the potty to watch George. I don't know if that made a difference, but he got to watch it several times yesterday!
Oh yeah, and don't be a tyrant about it like someone else is suggesting. Each child develops at his own pace. You just have to find what works for yours like I did. Making it an unpleasant experience is only going to prolong the process and make kids way more reluctant, if not scared.