I picked up the iphone 4 last month at Rogers.
They had a promo on (I think its still until the end of the month) with their data package its was $25/ 500mb or $30 for 6GB.
Needless to say i went with the 6GB.
I dont think i will ever come close to putting more than a small dent in it, but hey its there incase I do.
It can be one hell of a bill when you tether it to a pc and they start charging you per KB, like this poor guy.
http://www.msnbc.msn.com/id/22262994/