Aha. I thought I'd searched everywhere. Another useful thread.
I have the same problem -- I think -- as chippyb. I'm on Orange UK and get UMA on my home network (ISP idNet) which stops damn near everything from working (see Orange UMA in UK - working
Orange advised me to set the Connection Preference to 'Mobile Network Preferred' (amongst other things). The wifi logo greys out but I can browse using the wifi browser. BUT 3rd party apps use GPRS not wifi.
I don't actually want to use UMA for calls, but I need to have WiFi up for other things (MidpSSH is essential for me)