Assuming that is what your camera allows! (Another reason to upgrade
)
On my d40x, I set the AE/AF-lock button to be Focus, and then the shutter button becomes AE-lock (half) and shutter(full). This is the closest I can get to I@M's ideal. (Unfortunately, the AE/AF-lock doubles as "lock photo" when you are looking at the shots. If (when!) I chimp, I have to be careful when I go to take another shot. I always end up with about 10 photos "locked" on the card!