I like this theory, but would propose a variation:
The problem isn't necessarily that mobile devices aren't powerful enough to run browser-based apps. The problem may be that browser-based apps suck for other reasons. There are really only three ways to build one:
Plain old HTML pages over HTTP, with optional CSS. As has been said many times over the years, this is the IBM 3270 model with additional chrome.
Flash. Proprietary. Resource-intensive. No UI standards.
Javascript/AJAX. Better than it could have been, but relatively slow (modern JS interpreters are faster by leaps and bounds, but as usual their deployment rate is limited by the existence of IE), confusing to program, no UI standards, relatively poor tools (witness the recent complaints about Firebug), an evolving diversity of frameworks and practices that will take time to sort out. (The term AJAX is not yet five years old, and this stuff doesn't happen overnight.)
These problems, which have existed for years, have led some to predict that desktop apps would experience a resurgence. By and large, this hasn't really happened, because though many desktop apps are great they still suffer from some big problems: They're hard to install and keep updated (not as hard as they used to be, but the gold standard here is now the iPhone App Store) and they're dangerous (click on a phishing link that installs an app and you'll belong to someone's botnet).
The iPhone platform succeeds because: installation is a breeze, security is not much of an issue (every app gets screened by Apple; trojan-horse apps will, when discovered, be unceremoniously tossed out and their developers blackballed); there's a critical mass of customers that are on the same platform and using the same interface; there's a fairly coherent set of UI standards and practices as demonstrated by Apple's own apps; there's the full power of a native development language, platform, and environment that has been refined for years by dedicated engineers at Apple and NeXt.
I'm still not convinced that the iPhone platform will grow to the point that it poses a serious challenge to the freaking web. But the possibility exists. We'll see how well Palm succeeds in using web standards to sell mobile devices. We'll see how well and how quickly the browser platform continues to evolve.