As far as JavaScript frameworks go, you could use Sencha or jQuery Mobile or one of the other few dozen frameworks out there --- or you could roll your own.
I've been doing mobile web development for about a year now. I've done both PhoneGap and over-the-air mobile web/native apps (with embedded webviews that load HTML/CSS/JS from the web when requested). Both types of apps accepted by Apple and in the Appstore as well as in the Android Market. I would recommend rolling your own and not trying to imitate the look of native apps. One of the strengths of using HTML5 is that you can create a custom look and feel that doesn't look like every other app out there.
Since you mentioned Sencha and jQuery Mobile, here's what I think having used both:
Using Sencha is nothing at all like using HTML5. You don't even write HTML or CSS. You write JavaScript that uses Sencha's classes/methods. The look and feel very much tries to mimic native and they do a pretty good job at it...except that most widgets look like they were ripped from an iPhone, which is disorienting for users on Android. That said, you get good performance and a boatload of widgets/ui controls out of the box. The price for this is a large, multi-hundred kb framework and a commercial license.
jQuery Mobile doesn't really try to look native and it really doesn't "feel" native either. jQuery has it's own ui controls that more or less look identical on iPhone or Android. With jQuery Mobile, you do write HTML and have a more transparent view of that as well as of the CSS. But, you have to write HTML the "jQuery Mobile way" to utilize the UI built into the framework. You don't get as many widgets and ui-controls as you do in Sencha and overall, the look and feel is not of the same caliber either. But, it's smaller than Sencha, has your standard MIT license and is more easily modified.
Neither is perfect and both have pluses and minuses. If you know some HTML, CSS and JavaScript, you will absolutely get better performance and and a smoother look and feel by rolling your own light JavaScript for your app that does just what it needs to do.
If you'd like to see an example app I built using PhoneGap and handrolled HTML/JavaScript/CSS, you can download Easy Baby from the Android Market. It's free. You'll find a link to it on my website: http://newbabysleep.com