In Finder, Control-click on the application bundle (i.e. Microsoft Word), and select Show Package Contents from the pop-up menu. Drill down to Contents » Resources » en.lproj (or English.lproj as some apps may use, or whatever is appropriate for your language). Control-click on InfoPlist.strings and use the contextual menu to open it in TextWrangler, BBEdit, or TextEdit.
At the very bottom of the file, change both the keys for the display name you want. For example, for Word, I used:
CFBundleDisplayName = "Word 2008"; CFBundleName = "Word 2008";This changes both the menu bar name and the name that shows up in the Finder's Open With menu, and other places. Hope this helps someone out.
[robg adds: I believe that if you use this trick on a signed application in 10.5 or 10.6 (such as any of Apple's apps), it will break code signing, which may then cause some Keychain issues.]

