DHCP problem wireless only

Archived from groups: microsoft.public.windows.networking.wireless (More info?)

We're using a Cisco 1100 AP and have it running for a month or so fine.
We're using TLS with the Q815485 patch. Today, one of the laps won't get a
DHCP address anymore - it had been the last month or so fine.

I found a few tips:
1. enable UPNP - no change
2. Remove 815485 patch - can't as we need it for TKIP/WPA/TLS
authentication.
3. A few KB articles 320229 and 313896 mentioning versions of dhcpcsvc.dll -
the one installed is newer date and higher version.
4. Rollup Patch 826942 (superseded rollup including newer version of
815485) - no change

The RADIUS server authenticates fine and XP reports "Authentication
succeeded". The DHCP server reports :
dhcpd: DHCPDISCOVER from 00:0d:54:9f:b4:0d (WC0038) via eth0

dhcpd: DHCPOFFER on 172.16.6.74 to 00:0d:54:9f:b4:0d (WC0038) via eth0

so the packets are getting TO the dhcp server OK, but XP never gets ACKS the
lease and eventually times out. Plugging in a wired Ethernet obtains a valid
DHCP address - it's only wireless having problems. I can manually put in a
wireless IP address and it works fine - it just won't get a DHCP address.

What else can I check?