I'll be honest; I have a generally low opinion of fixed position toolbars in the mobile space. From a usability standpoint, they're a disaster. Mobile screens have very little usable space to begin with. To waste that much more real estate without providing a strong benefit to the user, is unthinkable. Moreover, because of the CSS involved, ancient versions of Android (less than Version 2.3) will not support the fixed position toolbar.
<rant>Yet, we see it all the time don't we? Companies slap their logo on a top toolbar that never goes away. They throw on a little global navigation and call it a benefit to the user when really it's all about them reinforcing their branding. You can tell because the only interactive parts on the bar are a menu button and possibly a search button (as if we couldn't have found them again at the top). There are many better techniques to provide global navigation. </rant>
Today, we have a valid use for...