Not quite. I have run into this problem too, only to find out that if the machine wakes from sleep, it tries to reconnect as WPA. The problem stems from a bug in the Network preference pane. It's easy enough to fix, though. In Terminal.app type:
sudo nano /Library/Preferences/SystemConfiguration/com.apple.airport.preferences.plist
Search for the string "WPA Personal" (it occurs twice) and replace it with "WPA2 Personal". Ta-da, it's fixed. I haven't tried to patch the Network preference pane to fix the problem, and I'm not sure that I'm allowed to. If anyone would like a script to apply the fix, I'd be happy to oblige.