I think it largely depends on what you want to do and functionality you’re trying to capture. If you want a webpage to behave a certain way on mobile and the same desktop, then with some careful coding/testing, you’ll be alright with jquery.mobile.
If you check out the CSS for jquery.mobile (uncompressed version), you can actually edit the code directly to show the HTML elements the way you want to for particular screen sizes. Just select the one for desktop / large screen sizes to scale appropriately.
Depending on what you’re trying to accomplish in terms of your mobile version, I’d also check out jQTouch – which allows for mobile-specific functionality, such as “tap” (in place of “click”), as an example – but also has a desktopCompatability option, so it reverts for desktop browsers.
One note, with either of these libraries, you’ll still need the root jquery library.
Hope this helps.