Great point. I started out with JQM early on and had to drop it. I ended up just pulling some of the touch event handling code from JQM since that was the only part still being used.
I can't really say, because we haven't done much testing on Android yet. My understanding is that Android's WebView uses a webkit rendering engine, but generally has poorer performance than the iOS UIWebView. So my guess is optimizing for iOS won't necessarily hurt anything for Android, but Android won't have quite as good of an experience. Hopefully this will get better with time... the UIWebView performance increase we saw from iOS 4 to iOS 5 is encouraging.
Unfortunately, the app is still in review with Apple, so not at the moment. I also didn't realize this post would do so well with HN. I will work on a more in-depth follow-up with some working examples. Thanks so much for reading!
sketchwith.us is actually a site I put together with a few coworkers... we didn't intend to publicize it yet, but since it is out there, please let us know if you have feedback