I think one of the big factors is how much you can use other connections. I confidently expect to shift several GB/month in and out of the N900, but at home it'll be on WiFi, at work it'll be on WiFi, and an awful lot of cafes and even some of the buses round here have WiFi.

I think that the amount of time that I'm reliant on 3G and still needing to move a serious amount of data will be minimal, but I can certainly see the high usage contracts being helpful for people that spend more time away from fixed locations with usable networks.