I've been working with jQuery Mobile for a week now and it's pretty cool just how quick you can get an app up and running...and most importantly looking great. Granted that's mostly to do with their CSS files. However, I really like how they designed their page navigations - in effect, the app acts as a browser within the browser and animates transitions between pages, auto-generates back buttons, lets you put footers with persistent buttons, all that stuff you've grown used to on phone apps.

As people have pointed out here, you can probably create 80% of the apps that your client would request with it and other HTML5 tools, rather than writing native code. Added bonus, it will run on iOS, Android, WebOS, and likely in the future Windows Phone 7.