This is a big mystery to which there is no real answer.
I have a I900 as well, but I do not have 3G/WCDMA in the US via AT&T which uses 850/1900 bands (which I believe is what Rogers and Fido use).
Here are some interesting bits:
- The I907 is a North American version that supports 3G on 850 and 1900.
- No one to my knowledge has seen any different hardware parts related to the Cell in the I907 vs the I900. The radio in both supports 8 of the UMTS bands that have been proposed (only 4 are in common use, and one of those is only in Japan). The cell antenae of the I900 are designed for optimal gain in the 850/1900 bands.
The Radio is idential to that of the Diamond, Touch Pro, G1, Kaiser, Touch, Touch HD and anything else using the 7201A or 7200 chipset. The I900 and I907 do not use this chipset, but it uses the same radio hardware.
No one was able to get the EU Diamond to use the 850/1900 GSM band, and in fact the US Diamond has the same radio Firmware as the EU diamond.
I was confident that if anyone could figure out how to make this work, it would be people at XDA on an HTC device (caveat - there are a lot of europeans here who don't care about 3G in north America, so I may have been wrong there)
Where does it say tri band UMTS on your phone?