I found this last night and have found no other mention of it online to date.
If you run Software Update, and you have updates that can be installed, here's an interesting trick. Note that you must be able to land on the screen after the 'check for updates' splash. Once you reach the screen where you can select an update, drag a QuickTime movie into the lower white portion of the software update window, where the updates' descriptions are shown. The window will play the movie inside it, as seen at right. You even get the green "drop" arrow when you drag the movie over the area. I also noticed that once playing inside, you can drag out a "clipping", just like with a regular QuickTime window.
You can also drop images inside this area, and they will display with scrollbars to allow you to view the whole image.
I discoverd this by accident last night, and right now, it does not serve any real purpose, but it is very cool, and it does seem to fortell future ideas that Apple may have about what happens at the start or finish of an update installation.
[robg adds: That's my screenshot there, and as you can (maybe) see, you get the full QuickTime controller, though you may have to resize the Software Update window to see it. I agree there's no practical purpose to this hint -- if you want to watch a movie during an update, just do it directly by switching apps -- but it is interesting.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20051102201155881