I'm not entirely sold that the right solution going forward is to use the toolkits we already have, but if you're building a mobile app today and you're going to use a toolkit, it looks like (as on desktop browsers), Dojo should be your first choice. The webkitMobile variant of Dojo points to one possible way out of the "what to do about mobile?" conundrum regarding size and IE-induced bloat, but I have some hope that WebKit will improve quickly enough to make the toolkits of today obsolete on mobile phones entirely. Time will tell.
Hat tip: the Uxebu blog