I did that once by accidentally setting an application hot key to be Shift-(and some letter key). Did you recently change hot key settings? If not, did you install something recently that may have had hot keys. If all else fails, try rebooting in Safe Boot Mode (hold down the Shift key on startup until the gear starts spinning), log in and see if you can type. When you are done in Safe Boot, restart w/o any keys.

KeyRemap4MacBook

^{I knew I had seen this somewhere…}

All you need to do is install KeyRemap4Macbook.

From the "implicit behavior" section:

Share state of modifier keys with all connected keyboards:

When you are using multiple keyboards, modifier keys are shared with all keyboards.

For example, pressing "shift key on keyboard1" and "space key on keyboard2" sends shift-space.

Yes, it works on other Macs too, not just MacBooks.