By default, the AirPort Utility only allows several 802.11n radio mode options as shown in this screenshot -- basically four different 802.11n modes: b/g compatible, 2.4GHz, 802.11a compatible, and 5GHz. To get four more options (b/g/a only, without n), just hold down the Option key while clicking on the Radio Mode drop-down menu in Airport Utility's Wireless tab. You should see a list of options just like this one.
This is documented in Apple's current Designing AirPort Networks PDF, but is somewhat hidden in the middle of the document as a side note! Maybe this is of help for someone trying to create an "n-less" WLAN.
Mac OS X Hints