Channel 13 is legal in the UK and many other locales, but not in the USofA.
Since most default setups use channel 6 or channel 1, those of us looking for uncrowded channels often find 13 is uncluttered.
When I did a recently clean install of Leopard, I found that my channel 13 network disappeared — invisible!
Update (2009-03-09) It seems that the 10.5.6 update (I installed it on 2008-12-27) has changed something so this tip no longer works!
Although my MacBook Time Zone was set to Edinburgh - Scotland (you can set this in System Preferences, under Date&Time), I found that my Wireless Card Locale was set to US (which disables channel 13).
You can check your Wireless Card Locale by going to "About This Mac" > "More Info ..." > "Network; Airport Card".
The solution was to set the Time Zone to Tokyo then restart. Wireless Card Locale is then "Worldwide" and channel 13 is enabled.
Setting the Time Zone back to "Edinburgh - Scotland" and restarting again gives me "Worldwide" — but setting Time Zone to "Los Angeles - USA", and restarting, sets it back to "US".