I have seen a very similar problem with orinoco equipment (lucent chipset, I don't know who makes the USR stuff)

Let me ask you this....is dchp being used? We found that if we set both the ap and the workstation with static addresses everything would work just fine.

If the AP was static, and the workstation was dhcp with WEP disabled, everything would work just fine.

If the AP was static, and the workstation was dhcp with WEP enabled, it would take about 20 minutes before the workstation would find the AP

If both the AP and the workstation were set to get an address through dchp and WEP was disabled.....we don't know how long it took for them to finally find each other, it happened sometime over night.

With WEP enabled, it took three days.

Strangely, when we took the ap to other sites, we had no problems at all. We concluded it was something with our dhcp here in this building, but as of yet, have not been able to track that gremlin down.