I seem to remember reading on this forum before that if you already have a blackberry activated SIM card on the Telus MIKE network, you can use any NEXTEL blackberry, no unlocking necessary.
Who cares? well, lets say Nextel gets the 7530 in 2 months (i'm making this up completely). I have a Telus MIKE network 7510. I would be able to get the nextel 7530 and pop in my SIM card, and it would work. Or lets say you break your nextel 7520, and you can buy one used and cheap from someone in Canada selling their Telus 7520. Putting in your Sim would work.
It's late though, so I might be wrong. Search the forums here, because I definately remember reading about it.