tentsu - You're right about entering the US zip code but you shouldn't have to do this. In fact, I think I did something similar when I set up the OTA tuner for SageTV a few months ago. Zap2it and other sites have Canadian lineup data for cable, satellite and OTA analog. It is about time that they recognized that we have OTA digital here in Canada! And it is not an issue of not having the channel data since all of these channels are available on cable and sat so they have that data. It is only an issue of having a database of what digital OTA channels are available in each city.
Chudsmith - The ideal situation would be if you could manually input the channels on the TiVo or do a channel scan. That way you might be able to use both cable and OTA. Manually inputting the channels is sometimes confusing with OTA digital but it is not too bad to figure out if you have a list of channels available in the GTA (which is readily available). But I am not sure if TiVo lets you do this. You might want to ask on some TiVo forums.
The other option is to build an HTPC with an OTA digital tuner and a analog tuner hooked up to analog cable (or even your digital cable box). You can do this with Vista Media Center and XP MCE (with a minor hack) or with other PVR software such as SageTV, Myth, BeyondTV, etc. I have a setup like this using SageTV with three tuners - an OTA tuner, an HD-PVR for HD cable hooked up to an HD cable box and an analog tuner (PVR-150) hooked up to my SD cable box. It is initially more expensive, more complex and it requires a PC by your TV but it is, by far, the most flexible solution. Unlike TiVo it also does not incur a monthly fee so it might actually be cheaper in the long run.