I happen to agree with Andy on just about everything in that post, but I do have some comments that potentially relate more to the Canadian market.
It's true that mobile costs are going down for users in the US but the opposite is true for mobile users in Canada. Here, the mobile carriers are not facing the same level of competition as the carriers in the US. Local overages and long distance usage is still at a premium and not one carrier offers an unlimited North American-wide calling plan, at least that I am aware of or has been published. Also, the iPhone is not being offered in Canada at all, so one would have to hack it to get it to work, which means a loss in some functionality along with the other side-effects.
This actually presents a larger opportunity then some may realize for middleware VoIP service providers / developers and not just for iPhone but for any Smartphone. In the next post, I will explain myself a little better and go into some detail about what some companies like Lypp are doing about it.