You need an unlocked device...
Most models of Blackberry and the iPhone 3G
/3GS are all easily available on ebay and other stores online (and in brick and mortar in most cities). Or you can buy one from a carrier and then buy the unlocker or unlock code online and unlock it yourself... iphone that's not so easy but Blackberry its just a few keystrokes... unlock codes run anywhere from $5 to $20 USD normally, sometimes you will find cheaper "giveaway" deals posted, but be careful as there are scams out there.
Just a question of what you want to do with it...
Personally i prefer Blackberry over iPhone because I use my device for email and txt more than browsing and like the full keyboard, i use a BlackBerry 9700 bold right now, great device but brand new so still a few bugs with it. Blackberry 8900 is a good device, solid, but not 3G
Once a phone is unlocked you can pop almost any SIM into it and get service... voice and data are dependent on the services assigned by the carrier to the SIM though.
Those who would give up Essential Liberty to purchase a little Temporary Safety, deserve neither Liberty nor Safety.