My son also had trouble with BMs (it's a common problem with NT children as well.) Sometimes he would close the basement door and go downstairs to poop. I happened to catch him and get him on the toilet- after that (including all the praise he got) he continued to poop on the potty and does it all the time now. What I'm trying to say is that timing is EVERYTHING!!
We now are fully trained and he never messes now.
Good luck to you
http://www.autism-pdd.net/forum/forum_posts.asp?TID=16098&am p;KW=potty+resources
It includes this reader friendly article about encopresis (poop accidents):
http://www.healthcare.uiowa.edu/cdd/patients/enco2.asp
If you search on encopresis on our forum (using the search button on the top of this page), you'll find you're far from the only parent dealing with poop accidents in an older child.
Good luck with everything! Oh, and a big congratulations on the success you've already achieved with peeing and wearing underwear.