Here are two test cases that have made me cringe lately:
You'll need to try it on the Android browser.
This one is with Bing Maps:
and this one with Google:
See the big red rectangle with position:absolute hovering on the map?
Notice how with position: fixed in the header that thing disappears?
I suspect some sort of z-index tomfoolery going on, which probably doesn't bode super well for the performance of position: fixed on Android. We can only hope for the day when ChromeWebView becomes available to us HTML5 app developers.