i'm surprised noone said any thing but:
Blackberry data vs. Iphone data has been debated since the iphone came out and the unlimited browsing on mobile phones (wap only!!)
Blackberry devices route all web browsing, email push, info push, and any other data requests from your handheld through their own proprietary servers, this in essence works as a firewall hence the vaunted "security" of Blackberry devices. This also compresses data that you use and i'm not sure the ratio but it is very minimal. FOr example, In any period with hard surfing and 9 active email accounts and various push services, IM that i have i rarely break 150mbs of usage. That being said i alos have a Rocket stick from rogers which uses the same data plans as an Iphone and for one week i used it instead of the blackberry and i easily hit over 1gb(in 1 week!). (using a rocket stick on an MSI wind, checking 9 emails frequently, sports, weather, various surfing and work related items, multi IM programs), thats not even including the other fun stuff i use the bb for like Texas Hold'em 3 Multiplayer!!and wordmole vs friends LOL
so you will be using more data with an iphone and the 6gb plan they had is no longer available but at least they(Rogers) capped the data usage to a maximum $100 /billing period
just imagine what the usage will be with the new OS coming for Iphone which will include their push services!(though i heard there will be some type of compression as well).
Blackberry data plans will not work with Iphones and vice versa.
i know this is a little late but i hope it helps