iBook prob with Cisco 1200 AP

I'm in the process of replacing 5 year-old Lucent Wavepoint APs to Cisco
1231s. Each 1231 AP is set up for 2 SSIDs (on 2 VLANs) using 40-bit
WEP. [We'll be going to 802.1x soon, but must stick to WEP for now.]

I've tested a variety of wireless NICs with the Cisco APs and all work
fine - save one. That hitch is an iBook with an Airport Extreme card.
It can authenticate to one SSID but not the other. Unfortunately, the
Mac does not give a meaningful error message ("there was an error
joining the Airport network..."). Oddly, PowerBooks with Airport
Extreme cards don't have this problem.

Just wondering if anyone has run into this before and managed a workaround.

Thanks,
Tom