I have this same problem. After some work (using install disk & repairing the partition, then playing with the size of the attempted bootcamp partition) I found that the problem is [apparantly] some files are too large to move.
My xp virtual hard drive for parallels is about 10GB, maybe that is the culprit.
I guess that the journaled HFS does not defrag (well) as it goes. I cleared some 60GB to prepare for bootcamp, and still no luck.
Now I am organizing all my backups. The plan is to clear off an external hard drive, turn on time machine, load up any free space on the time machine drive, and try again.
I figure that by the time all this backing up is done I can actually wipe out the hard drive on the mac and reinstall leopard, etc, and do the bootcamp thing.
I figure that not having to do any maintenance for so long since I switched that it isn't the end of the world to go through this hassle.