There have been a few articles recently about Google\'s new imageless buttons:
The majority of the work here probably won't be the design - those posts are already an excellent How-To on the gradient effects.
The problem is getting this working in all browsers, or more specifically the quirky piles of rubbish that are IE6 and IE7.
I think you're on the right track with a standard button that gets re-written with jQuery - that way you'll still be accessible for screen readers and can degrade nicely in really old browsers.
For the HTML I think your best bet is to visit Gmail with each browser and see what HTML is produced - I'd expect it to be completely different for IE6, IE7, (also depending on whether they need quirks-mode) and everything else.