I'm still not an iPhone user, but I'll be waiting until a multi-core version comes out. The 3GS seems to be a great device, and definitely was worth the upgrade, but it isn't enough to warrant:
switching carriers to AT&T, and starting another contract.
paying for a data plan. I'm not currently doing so, but will once Apple produces a device that I feel is worth $30/mo.
If the next iPhone is multi-core (and possibly if it isn't), I will make the switch. I can't even imagine all of the possibilities that could open up for the Apple team.