The very last place I recommend browser sniffing code is mobile. It's the wild west out there right now and nobody is reading from the same rule book. I've heard rumors some browsers are mimicking other user agents to avoid sniffing code that excludes them just like IE used to back in the '90s.
Test by method and google progressive enhancement if you don't already know what it means. Some browser implementations of JS are so messed up they're not worth trying to accomodate and should be served versions of a site that work without JS, IMO.
Just look at what JQuery's core team considers C-grade (not worth supporting) on their graded compatibility tables. And yes, I believe this applies to both the core and the JQ mobile library (which is kind of like JQ ui for mobile, not JQ core for mobile - still needs core to work).
http://jquerymobile.com/gbs/
