
Want to see the date in the menubar, as seen at right? Start by opening the International Preferences panel, and clicking the Formats tab.
In the Dates section, click Customize. Click the Show pop-up and select your choice of Short, Medium, etc. Now place the elements you want in the small "work area," by dragging and dropping them from the Date Elements section. Once placed, you can click on some elements to see other options (Wednesday or Wed, for instance). Once you've built the date string you want, click once in the work area, and hit Command-A then Command-C to select the string and copy it. Now click Cancel to close the window without making any changes.
In the Times section, click Customize, and set the Show pop-up to Medium. In the small work area below the pop-up, place the curser where you want the date elements to start, and click to position the cursor. Now hit Command-V to paste the elements you just copied. You can further add characters and spaces to customize the look. If you want to remove an element (i.e. the year), click on it and use the Delete key. Click OK, and you'll see the results in the menubar.
[robg adds: There are some older hints that explain how to do this for pre-10.4 systems (10.3, 10.2, 10.1), but this is by far the easiest version I've seen yet. After some frustration, I realized that the settings in the Date & Time panel apply to the clock -- so if you have "Show day of the week" selected there, then it will always appear with the time, regardless of your settings in the International panel. To get rid of the date, just edit the Medium clock format and remove the fields you added. Finally, only the Medium clock setting will show your changes -- this is the one that OS X is using to display the clock in the menubar, it seems.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050508000838365