3GS is a great phone. I'd be interested in knowing what firmware it is running. 4.1 or below is good, 4.2 isn't optimal. The only downside of having an unlocked iPhone is that you have to be up on jailbreak news. When you plug in to itunes, you can't just click "upgrade" when it tells you to, else you could end up with a phone that can no longer be unlocked. It happens all the time. If you are not a techie person, you should probably just wait for a Rogers branded phone.

As far as the warranty goes, you can always just to a restore and the device will be back to an unjailbroken speck. Apple would be none the wiser.

The only jailbreak that exists for 4.2 right now is a "tethered" jailbreak. What this means is that if your battery goes dead, or if you need to reboot your phone, you will need to attach it to a computer and reapply the jailbreak. Not a good way to set up a device that is away from your computer most of the time. Even the jailbreak team is recommending staying at 4.1.