Did you originally make the partition with Boot Camp and are now trying to resize it?
To the best of my knowledge (and I may be wrong) you cannot otherwise change the partition table on your boot device. You could boot from the installer DVD and format the drive, but short of that, I do not think you can do what you want...
You *might* be able to reabsorb the partition and then use Boot Camp to create a new, larger partition, which you could then format HFS+, but I'm speculating wildly at this point.