Currently, the 'mobile' site using jQuery Mobile loads in desktop browsers having size 1280px or lower. As a counterpoint, tablets (and mobiles) will continue to get viewports with larger dimensions and having a reduced breakpoint is also counterproductive.

Ideally we'd want to have device viewport detection logic that:

is simple for beginners to use (since these are quickstart)
is mostly reliable and typically used in production
does not rely on browser detection and similar hacks
relies on feature detection (features should be enabled in the quickstart when they are available on devices).

In our quickstarts using jQuery Mobile, we're more reliant on detecting touch support or lack of it (using device viewport widths as a proxy). In the absence of reliable detection for touch, it would be better to demonstrate responsiveness.