Why is my S key intermittently typing ß inßtead of s <----See
You can check to see what OS X thinks is happening when you press the key by opening the Keyboard Viewer applet.

First, go to Language and Text in System Preferences, to enable it:


Then select 'Show Keyboard Viewer' from the Input Sources icon in the status bar.

Now, when you press the key, you can see which combination of keys OS X thinks is being pressed. You can use the keyboard viewer to enter text, too, in order to work around the problem:



You can confirm this way that it is indeed a hardware problem.