This was posted over on Hofo, credit goes to that poster:
HowardForums: Your Mobile Phone Community & Resource - 8320 Stable Wi-Fi UMA Connection Setting

== snip ==
This may help:

Per LINKSYS tech support and RIM use the following values in your router:

DTIM: 2 (1-255)
Fragment Threshold : 2304 (256-2346)
RTS Threshold : 2304 (0-2347)
Beacon Interval : 50 (20-1024 ms)
Channel = 3 (or least used channel in your area)

This has solved many issues with failed UMA connections.
== snip ==

I am not a big UMA user and/or have tried, just passing on some info.