Persuade Airport Extreme it's 802.11b rather than 802.11g
This is useful if your wireless access point is old and requires a firmware upgrade... and of course if you don't own the wireless access point (such as though used in wireless hotspots) then you won't be able to upgrade the firmware...
Some 802.11b access points require a firmware upgrade before Extreme (802.11g) clients can connect.
Though 802.11g is backward compatible with 802.11b some older access points were developed before 802.11g and therefore are not in a position to allow the 802.11g clients to connect.
Check the vendor site for a firmware upgrade for your wireless gateway.
Sometimes you can't upgrade the wireless access point (as it's in a cafe for example).
Note that you can attempt to persuade an Airport Extreme client to operate in 802.11b mode.
Hold down the control key [ctrl] when selecting the network from the menu.