On occasion, after putting Word v.X or Excel v.X in the background, I'm unable to switch back into the application. Instead of the expected window switch when I click a portion of a visible window, I'm greeted with a "beep" and no switch. It turns out that the nearly-vanquished "modal" dialog box is to blame. Modal dialog boxes are those that, in OS 9 at least, cannot be moved to the background until they are dismissed. Office v.X uses them for its Print dialog boxes, and perhaps in other locations.
In OS X, of course, almost any window can be moved to the background. So there's no issue switching out of Word or Excel after opening the Print dialog. However, the modal dialog box retains its "modality" within the application from which it was called. So to the application, that dialog box is in full control, even though you're now using another app and have probably covered the dialog box with other windows.
When you go to switch back to the original app by clicking on a visible window, you'll see the behavior I described - a beep and no switch. This is because the modal dialog box is waiting for input, even though you probably can't see it. Since you didn't click inside the dialog, you get an error beep, just as you would if you tried to click outside a modal dialog box in OS 9.
The hard way to get back to your original application is to move or hide any of the windows that block the modal dialog box; once you can see it, click in it to re-activate the original application. The easy way to do this is to click the app's dock icon; this forces the modal dialog box to the foreground as it switches back into the application.
I'm not sure if this affects Entourage and PowerPoint, but it definitely affects Word and Excel.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20011217012330663