and my 2 cents...
We found ExtJS on tablet just to heavy weight with dom structures and took to long to render - that is because our app is heavy with components and features. A light weight app may work just fine delivered as ExJS on a tablet.
I cant see why anyone would use ExtJS to deliver to mobile - UI interaction is completely different due to screen size issues.
We have adapted Touch to deliver a desktop style app to the tablet and base Touch components to deliver to mobile.

this may help with specifics with differences - http://www.sencha.com/forum/showthre...xtjs-and-touch