How to get external Apple keyboard to accept modifier key input from Macbook's internal keyboard (and vice-versa)?
KeyRemap4MacBook
I knew I had seen this somewhere…
All you need to do is install KeyRemap4Macbook.
From the &quot;implicit behavior&quot; 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 &quot;shift key on keyboard1&quot; and &quot;space key on keyboard2&quot; sends shift-space.

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