The 75xx and 65xx phones should roam into Telus and NII in Mexico and South America for voice service. Telus offers iDEN Blackberries so there is a good possibility that you can use data on the Telus network from the US (or vice versa)

NII doesn't offer a Blackberry so iDEN blackberries might not have full functionality in their network. (I guess you'd have to look at the NXTL to see what services are showing)