As product lines, the BlackBerry and iPhone really are aimed at users with a different focus. I don't think you can compare period, but no doubt you can't compare unless you state your must-have functions and features.
And correct me if I'm wrong but the iphone 4 or whatever name they give is gonna have "selective" multitask. Meaning that will allow music, GPS, and some other third party task running in the background but then again limited. Its certainly a step up but not entirely as free as multitask on a BB or an android phone
I considered switching from my old Curve 8330 on Verizon to an iPhone but the plans were so expensive for what you got I decided to just stay with Verizon. I ended up with the ability to upgrade a few days ago so I went with the Tour and I'm a happy camper now. It's alot cheaper than the iPhone and sports similar features minus the larger screen and touch capability.
I have 4.0 beta v3 on my iPhone (that I am currently selling for a 9700). IMO the multitasking is nothing to rave about. I played with my friends 9700 this past weekend and I totally enjoyed the interface on the 9700.