How would a phone like the Samsung I7500 upgrade its firmware when running on a non google partner carrier? I don't see any kind of manual upgrade options available.
Most phones have a way to flash the ROM from an SD card. You will be at the mercy of the manufacturer to release the upgrade ROM files. That hasn't happened much in the past. You could also use a carrier ROM if you could find one of them.
It could flash from SD card or flash over a USB cable like my Openmoko can. Heck, it could flash OTA anyway... if it's got net (via 3G or wifi) what makes you think it needs a 'google partner carrier'?
Just FYI, I purchased a G1 off craiglist from someone who had disabled their T-mobile account before Cupcake (Android 1.5) was released. I then proceeded to unlock the phone and use it on AT&T (I know, no 3G and all that, but I couldn't wait for my AT&T contract to end before getting an Android phone). When I started up the phone the first day I owned it, I believe it had firmware version 1.1 (forget the exact version but it wasn't Cupcake).
The next day when I turned it on it said there was an available firmware update it wanted download (this is while connected to AT&T). I connected it up to wi-fi so the download would go faster, and let it update. When it finished updating and rebooting I had Cupcake installed.
So the moral of the story is, I don't know what it's hitting to get firmware updates, but it's just some internet server, it's not required to be on T-mobile's network to get the updates, it just needs to be connected to the internet.