If you want cross platform, web based, mobile support for devices beyond only webkit browsers (iOS, Android, BB 6.0+), you need a scalpel, not a chainsaw. Sencha, jQuery Mobile, and the like are great and give you tons of functionality right out of the box, but as many mentioned, they are mostly catering to the big mobile players and will likely leave you wanting when it comes to supporting and lower end devices.
My choice of scalpel has been phonegap + xuijs and I'm loving it so far. While it is a bare bones JS library, it gives you the foundation to build a reliable, gracefully degradable experience across the entire spectrum of smart phones (with wp7 on the way). Write your transitions, skinning, etc... as CSS and browsers that don't support them will simply deliver the content without the flare of sliding views, round corners, gradients, shadows, etc...