Yeah I really hope jquery mobile can match the smoothness and fixed menu support to be on par with Sencha.
That's definitely the main thing keeping me on the Sencha side for now too.
Considering how new this project is still, the progress is very encouraging though.
While I love the html syntax for JqueryMobile (Sencha is a giant Javascript function, which had a steep learning curve at first) I felt that the widgets and display quality of jQueryMobile just didn't match that of Sencha.
Sencha's transitions felt faster, animations were smoother, and form items felt more like native parts than in jQueryMobile.
I eventually went with sencha because it produced a better experience on a tablet, but I think I would definitely like to give jQueryMobile a try on my next smartphone project.