Here is an issue I ran into the other week.  I wanted to add VLAN's to the three SSID's I had already set up on some Cisco AP's.  I already had WEP setup on two of them, and a WPA Pre-shared key set up on the other.  When I created the VLAN's and applied them to the SSID's it cleared out the WEP keys and the WPA key.  In order for clients to reasscoiate with the AP's I had to reenter all the information back in.  Just an FYI.