Numbers '08 seems to have an overly limited list of date and time formats. I'm sure they're enough for most people, but my aesthetic sense demanded one that wasn't listed. It doesn't look like you can add other formats within the actual program, but there is a plist file you can edit. (I've also posted this on Apple's Discussions site.)
Quit Numbers if it's running, then control-click on the Numbers application icon and choose Show Package Contents from the pop-up menu. Type date into the Spotlight field in the Finder window, and click on the DateTimeFormats folder. Alternatively, drill down into Contents » Frameworks » SFUtility.framework » Versions » A » Resources » DateTimeFormats.
Control-click on en_US.plist (or your language's plist file) and click Duplicate to make a backup. (Just rename the backup if you damage something.) Open en_US.plist (or your language's plist file) with a property list editor. I use Property List Editor from the Apple Developer Tools, but I think Pref Setter is also easy to use. Expand AcceptedDateFormats and check if your desired format is there. If not, add a string key with the desired format. Open DisplayedDateFormats and add a string key in the format you want the date displayed.
Save the plist file and restart Numbers -- your desired date format should appear in the list of formatting choices. (This also works for time formats.)
[robg adds: If someone manages to find the counterpart to this hint for custom number formats, please submit it! I searched quite a bit, and didn't see anything that looked like it would work.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2007081022564738