S. - I went through this exact same struggle with my son. At the Christmas break while my husband was home for my support, we finally took away the pull-ups. This was about Dec. 28th. We also took away toys and reacted negativly to his stubborn refusal to cooperate.
What finally worked for us, was calming down (mostly me calming down), sticking to the No Pull-ups during the day policy. We calmly made him responsible for cleaning up his 'accidents'. Then when he was starting to have less accidents, about the time of Jan 6th, we started rewarding him with matchbox cars if he stayed dry all day while Daddy was at work he got to pick out a car. We also told him that he would get a big boy bike once he was totally potty trained. We visited the Spiderman bike at Wall-Mart frequently and talked about getting it for him. Lastly, I would call my Mom & MIL everyday to tell them how great he was doing being a big boy, using the potty - all the calls within my son's earshot. We started giving him high fives and double high fives when he pooped on the potty.
Also, when it was time for his pre-school to start up after the Christmas break, I told him he could not go back, until he didn't have accidents - he only missed one weeks worth of school.
I don't really know if any of the above made a difference for my son, or if it just finally clicked for him. I can tell you that when he finally 'got it', it was so wonderful for me - I have been bragging to everyone I know. My guy's 4th birthday was Jan. 7th and he stopped wearing the nighttime pull-up shortly after his birthday - he has not had one accident at night.
This might sound extreme to you, but if you are going to take the pull-ups away for good, you might consider taking a week off from work to really work with him on this. Maybe your husband could take the next week off, so he has two full weeks to really get it down before he goes back to his child care provider.
I hope it clicks for your son soon. Believe me I know how very frustrating this is for YOU!
W.