I don't know how the WPA key is stored on the phone itself, i.e., I don't know if they save it in a clear text file in memory.

However there is currently no way from its normal user interface to view the key after it's been typed in. (You can't even view it WHILE you're typing it in, which was Bruno's complaint.)

If you wanted to prevent a nicked phone from being used on your WPA network, you can PIN-lock the phone's UI, but I think that an "awake" phone will still connect to a known WPA network in the background, with the anticipation that someone will want to start using it as soon as they enter the PIN.