I have been using matrix metering. My mistake. I tried using spot metering tonight and got ae to lock. Seems to be intermittently not working tonight. Yesterday, I couldn't get it to lock to save my life but I think I had in matrix metering mode the whole time. The shutter button does not have the definite "half pressed" that I'm used to on my d50 but I am pretty sure I pressing in far enough. Maybe the slide switch is going bad. I just wanted to make sure there wasn't anything obvious that I missed.
Also does the af area have any affect on the ae-lock?