My customer site is having issues with 802.11b clients not associating to a WLC 4402-50 controller that is setup for both 11b and 11g. 802.11g is working fine. As a test, the client's wireless mode was set to 802.11g at the user WIC Settings. But, it still showing connection failed. Also found that the user MAC still coming in the Controller as 802.11b.
Just to add a note to the good info from Richard. Are you running any 11b supported data rates like 2.0 5.5 6.0 9.0 11.0
Note The client must support the basic rate that you select or it cannot associate to the wireless device. If you select 12 Mbps or higher for the basic data rate on the 802.11g radio, 802.11b client devices cannot associate to the wireless device's 802.11g radio.