I was also having lots of issues with UMA on Rogers, but got it working now. I upgraded to 126.96.36.1994 straight with App Loader (found on XP at c:\Program Files\Common Files\Research In Motion\AppLoader\Loader.exe) without wiping out my 8900, I even didn't have the official 188.8.131.52 but 184.108.40.2060 (Avea).
I read somewhere that by rebooting without Wi-Fi on and only activating Wi.Fi after the device comes up UMA would work, but this only worked for me last night. Today I had to reboot and UMA did not come up all day, not even soft (alt-shift-del) and hard reboots. I followed one of the comments, turned off GSM and Wi-Fi and only activated UMA and it came up in about 5 seconds, oddly enough.
On the 3G-UMA issue, T-Mobile offers "Wi-Fi calling" with the Bold and they do have a 3G network. When the 9000 came out for Rogers I heard that the strategy was that these 3G devices were more for "business people" and that Rogers had deactivated UMA on purpose (having said that, would it work to load the certificates and activate it through the e-screens, anybody?)
Hope this helps