I honestly haven't seen much of the Galaxy S, and I think it's too early to speculate how Rogers will have it customized.
If you are focussed on the ability to upgrade, I would strongly recommend getting a phone from the US rather than Canada, as it seems that the american carriers are more willing to allow updates.
So far, I've only heard word of the Acer Liquid E getting 2.2, and 2.2 is on most phones in the US already. So if you are truly set on new OS updates, I would steer clear of the Rogers Android phones at the moment. I cannot vouch for TELUS or Bell customers.
The only reason I'm so harsh is because with the Dream, Eve, and potentially the X10 (too early to tell), we were told of upgrades to come, and then got shafted. The LG Eve isn't even getting 1.6 until Christmas of this year, or even later.
However, Android 3.0 might be an upgrade similar to iOS 4, where new features are being implemented with MUCH larger system requirements. If that is the case, then having a device ship with 2.1 may only have a lifespan until 2.2. Of course, again, too early to speculate.
Here's the good news. I believe with the iPhone plans being no different, you could simply buy the Android device of your choosing, and continue on the same plan month-to-month. You'd lose the contract-discount, but you'd have the same plan
.
The only phone right now that really seems to be awesome is the Acer Liquid E. But, comparing that to almost anything in the US, it's not that great.
Any particular features you'd be looking for in your upgrade from the iPhone (literally, even my Dream is an update over those things )