%a %b %d, %l:%M %p
From reading the date manpage, you know that this will display the Year, month, day, hours and minutes. Something like this: Sat May 01, 5:16. But what about seconds? If you want that, just add the appropriate specifier (%S) to the date string:
%a %b %d, %l:%M:%S %p
Note: You will also need to check the 'show_seconds' box roughly 11 places below the date string setting in gconf-editor. If you forget this part, the seconds portion will display, but the numbers will not actually change until the minutes portion changes because the clock isn't being updated every second.