This is relatively useless and quite pointless, but the fact remains that:
If you open the Photoshop Color Picker dialog (by clicking either the foreground or background color in the Tools palette), some people may already know that if you have an open Photoshop document, you can click anywhere within the Photoshop document, and (keeping the mouse button held down) drag the dropper outside of the document to get samples from the screen around it (for example, from a webpage in the background, the desktop, the gray of a metallized app, etc).
Now, adding on top of this is the fact that, while the widgets themselves (the red/yellow/green Aqua buttons) of the Photoshop document in question turn gray/inactive in Photoshop while the Color Picker dialog is open, you can still do the aforementioned click and drag of the eyedropper to sample out colors of the widgets, despite the fact that they're showing up as gray/inactive to you. What's weirder is that while you're dragging around looking for the right color, the "current color" sample will show up as a value of gray, corresponding to what you see on the screen. Letting go of the mouse button will yeild the color in question (the red/yellow/green you're trying to get).
Relatively useful when trying to re-create an Aqua button for a web icon, or something equally pointless. Thought I'd share.
[robg adds: This doesn't seem to work in Photoshop Elements...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040112162134636