A Rogers representative told me that using any smart phone (iPhone included) on a BB plan is fine. He said he does it all the time as he has a BB and an iPhone. He said you cannot use a BB on an iPhone plan though.
as nthnc said, you can use a Iphone on a BB plan, but not a BB on an iphone plan...
however...you can still use a BB on an iphone plan but you will not get any BIS services...you can still use the browser, just no email/bbm/etc. If you want to use a BB on an iphone plan, you will need to call the data dept and get a APN3CON code put on your account and you will be good to go!