I tried out the 'Sexy Dropdowns' tutorial over the weekend. The js/jQ code turned out nice and clean, and I really like the fact that the solution gracefully degrades on non-javascript devices.
As mobile devices become more capable, and mobile browsers become more common (and hopefully toward ACID compliance), it makes sense to have a good fallback. It's getting tougher over time to maintain multiple code bases (PC -vs- mobile), and it's also a wonky kludge to inspect headers to decide which version to render.
Activating 'advanced features' via javascript seems a good, non-invasive approach, and I'll probably recommend retrofitting legacy Ruby on Rails applications toward that goal via jQuery in the future.