My DS turns 4 in May and up until last week was TT fine for #1's (for a the past year), and also fine for overnight without wearing a nappy, but had absolutely refused to do a poo anywhere but his nappy (which he used the same time every night.
Last Sunday we said thats it, NO more nappies for poos, and encouraged him to use the toilet, and then the potty - as he seemed more comfortable with that.
He did not then pass a BM until Thursday, which was lumpy and hard and involved lots of crying and screaming on his part.
Same thing happened Sat, and Sun, and yesterday I was called to pick him up early from kindy because he was so uncomfortable he wouldnt even sit down.
He spent the whole day/night crying, and lying on his belly with his bum in the air as this was the only position where he obviously didn't feel the pressure of the poo. He didn't even want to sit on the potty, and was just desperate to hold it in.
He was also getting leakage.
I had been giving him occasional Parachoc (paraffin oil) earlier in the week, so I gave him more, and began to ply him with lot of pear juice too (by this stage he didn't have an appetite either). and he STILL held it in all night.
This morning we gave in and went and bought some more nappies, and lo and behold a few hours later he had filled it.

I am now not going to attempt poo training until his bowel has returned to normal, and am going to have to find out how to approach it differently. Because he just downright refuses to let it out... after doing some research on the net I am finding out that it is not entirely uncommon for kids to go through this.... but my god... what a drama!

I don't have any advice, but just wanted to share, as I was going to post my own thread about this too...