Yup, no problem there.
Not all plans lock you to a specific carrier, you can purchase unlocked phones; the drawback is they cost a premium compared to the subsidized, locked phones from a carrier
Make sure your BB is 3G, as the some versions of BB sold in GSM/3G countries work only on 900/1800 GSM and not on the 3G bands here in Canada. FYI here are the GSM/UMTS bands used by carriers in Canada:
UMTS 850/1700/1900/2100 (AWS - 1700/2100)
I did not include the CDMA bands here (Koodo for example)...
The iphone does support these GSM/3G bands so no problem bringing an iphone (as long as it did not come originally from Verizon which is CDMA).
Here is a link for carriers plans in Canada:http://www.cellphones.ca/cell-plans/carriers/
/hope this helps....