Perhaps someone has already noted this, but Microsoft lists (in the Help file) a number of known issues with Office 2008. Some of them are fairly significant, and may explain some troubles you may be having with Office 2008.
You can read the full list of issues on Microsoft's support site, where they have a page of known issues for each program: Word • Excel • PowerPoint • Entourage
Finally, I uncovered one more issue while posting this hint -- the Help window can become unresponsive or just disappear all together while trying to clip out known issues!
[robg adds: As submitted, this hint listed every known issue in every Office 2008 application. I replaced the long list with the four links above in order to shorten the hint (there are a lot of issues across the four apps). While many of the issues are minor, some are not. In Word, for instance, you can't see equations created in Word 2007 for Windows. In PowerPoint, it's possible to open the same presentation in both PowerPoint 2004 and 2008, and have both open as fully writable! Finally, if you use (or are trying to use) AppleScript and Automator in Office 2008, one final known issue will affect you: "Running Automator workflows and AppleScripts from the Office script menu is not currently supported under Mac OS X v10.5 (Leopard)."]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080117144249295