When you insert a blank CD or DVD, a dialog box pops up asking what you want to do with it, giving you the option to name it if you want to get it ready for copying and burning. A couple of times now, this dialog has gotten "stuck" on me, when it pops up at a time when I'm navigating Finder windows to find the stuff I want to copy to the CD. I get a spinning beachball when the cursor is over the window itself, I can move the window, and use the Finder or other applications (and even use Disk Utility to eject the blank CD), but I can't dismiss the dialog.
Logging out and back in will fix the problem, but there's a less drastic solution. Open Activity Monitor, sort the list by %CPU and find the process 'SystemUIServer.' (Or use the filter box, but sorting by %CPU is how I found the culprit.) Quit the process (use the Force Quit option); the pesky dialog will disappear and SystemUIServer will relaunch itself immediately -- it also controls the menubar clock and fast user switching menu, and a couple of previous hints have mentioned SystemUIServer as the cause of problems in those areas, so add the "inserted CD/DVD" dialog to that list.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050926094719550