You say you just got the 12", presumably bought used...
Were any of the internals replaced at some point in the machine's history? New harddrive, mayhaps? I would check that the Airport Extreme card is properly seated and that the antenna is securely connected. Just a thought.
The airport signal on my 12" 1.33 GHz is solid. Has been since day 1.
I know this is old, but I was messing with my AirPort card the other day for the first time in months.
I was in the living room the next day and noticed the WiFi was connected with full strength. I don't know what I did or how I did it but the WiFi now works great on this laptop. No more having to use a USB WiFi adapter.