got an iphone 3gs with unlimited everything fm att. Can i switch the sim card...

[r0xas13]

...to a blackberry? love my iphone but at times i wanna switch it up and maybe txt people with a blackberry and use a blackberry. could i make calls, text, and go online if i switch the sim card of my iphone into a blackberry? would i have to go through a whole activation process everytime i switch back and forth? thanks