All you need to do is upgrade another line on your account (not one with unlimited data), upgrade to the iPhone 5, add a 2GB for $30 data plan if the line doesn't have one already, then activate the new phone on this line.
Then AFTER its activated, you can swap the iPhone 5 to your unlimited line and put the previous phone back on the line that was upgraded. If it is a non-smartphone then the 2GB data plan you added during the upgrade can be removed. A new sim card will be needed to put the newer phone on the unlimited line.
I think people are making the mistake of trying to transfer an upgrade from another line to their unlimited line which shouldn't be done if you want to keep the unlimited data plan.