To do this, open System Preferences and go to the "International" system preference pane. Once there, click on the "Formats" tab and then click the customize button in the dates section. Select "Medium" from the drop-down menu. You should see the data change in the text box below the drop-down menu. Click inside the box, select all the text (Command + A), and then copy the text (Command + C). Once you've copied the text, click cancel.
Click on the customize button in the times section. Select "Medium" from the drop-down menu. Then place the cursor just before the time and paste (Command + V) the date that you just copied. You might want to add several spaces (or some sort of separator) between the date and time. Once you are finished, click OK. The changes will now show up in the menu bar.
Update: To remove the custom formatting from the menu bar, go to System Preferences > International > Formats. Click the "Customize" button in the times section and select "Medium" from the drop-down menu. Delete the additional text that you add and click "OK." Now click the customize button under the "Times" section again and select "Short" from the drop-down menu. Click "OK," and your system should return to normal.
More tips and tricks like these can be found at the TUAW Tips and Mac 101 sections of TUAW.