A few comments:
Scalability: working from the lowest common denominator
Maybe it is ignorance on my part, but it seems like there is a much bigger investment in time for a site that serves up a set of pages that will scale well on a cellphone browser and still look world-class when served on desktop PCs. I would think it might be cheaper for the web site owner to pay for a different set of pages for the micro browser crowd. (My gut reaction to the current crop of micro devices is that they are "quaint", like a Commodore 64. However, it probably won't be long before they grow up, and then web developers will not only need to include them as the lowest common denominator, we will want to.)
What about the site visitor to your ecommerce site that prefers to surf with cookies disabled? I suppose there are ways to code everything server side, but again, this extra development time has to be paid for by the site owner - are they getting a smart return on their investment?