Actually, they legally can't in Canada.

From http://techreview.eatuniverse.net/2010/01/24/rogers-disconnects-data-services-to-g1-users/ :

"Some of you may recall legal precedence for this kind of issue, dating waaay back to the 70’s when Bell (then Northern Telecom) tried to force its customers to use Bell branded phones, and tried to attach a special fee to modem/fax use. The courts ruled against Bell, and the CRTC jumped in and made all sorts of rules to prevent telecom’s from requiring proprietary hardware, on standardized networks. This does, legally, carry over to GSM carriers. GSM is GSM. If your phone supports GSM on the frequency of a carrier, the carrier must provide a signal. Period."

I'll add a note that this approach isn't 100% proven. We don't know for a fact that the mods are sending the same information to Rogers contained in the build.prop files as the Rogers firmware is sending, or that the mods are even sending that information at all.

However, I do endorse this approach for lack of better ideas, and my phone is connected in spite of being on CyanogenMod.

I'm not sure if the network reset itself or the build.prop fix is responsible, but I'll take what I can get.

"will not work on Telus in those modes"

2g = won't work. 3g = will.

Agreed!

I'll add that there has been some luck fooling it by installing the CursorSense 1.2 mod, which is based on the Mandatory Update itself and reports the same or similar versions of just about everything.

Personally, I'm not that keen on switching mods unless I absolutely have to, though I do rank it as a better option than going with the mandatory update firmware.

So far, we're not altogether sure of what's being scanned for exactly. If it's a message the radio is sending out on connection to the network with a radio version, it'd be very difficult to intercept.

There's been suspicion it's just the build version, but build patches haven't proven themselves to work (it's been tried).

As long as the 3G phones support the correct frequencies, they will work on both networks.

Telus: 850/1900Mhz
Rogers: 850/1900Mhz
Wind: 1700Mhz

So between Telus and Rogers 3G will be compatible, but phones that also support EDGE and GPRS will not work on Telus in those modes as they lack the legacy infrastructure.

I've documented everything I've collected on the issue here: http://www.litui.net/archives/796

More detailed and ordered than the OP's links.