Fido does not support the BlackBerry.
Data will flow over the Fido network, but you cannot get a web account from them, and they cannot provision the BlackBerry for BES use (actually, they can't provision it for anything). So, if you get an unlocked BlackBerry and put your Fido SIM in it, all you get is a phone. I have an unlocked 6230 with Rogers, if i put my Fido SIM in it, all I get is voice....actually, since Rogers flipped all the Fido towers in my area over to Rogers towers, I wonder....testing now...."Data Connection refused". My SIM is not provisioned for BlackBerry data...and this is because Fido does not support it.
So there ya go, first hand testing done as I typed the reply