I talked with a Rogers rep today about the $50 rebate. Turns out it is not 'mail in' at all. Once you get your phone, log into My Rogers and go into "Products & Services". Under the "Wireless" tab, select your phone number and you should see a link labeled "Redeem Rebate" underneath the picture of the phone.
The link is not there now, but I assume it should show up on launch day. Hopefully what he told me isn't a bunch of BS. My last upgrade was three years ago (iPhone 3G), so I (and others who are eligible to upgrade) should be eligible for the new customer prices that are advertised.