Have a key action you want to highlight for users? You could takeover the application as Facebook did to promote Places. You could create a persistent notification bar as Quora does. Or you could make the key action button really stand out... as upcoming application Instagram does (read about Instagram on TechCrunch. They are a Dogpatch Labs company.)

Instagram is a photography / photo sharing application (download it in the iTunes App Store)... and the most important action is sharing photos. So while there are other important actions (which all get buttons across the footer: feed, favorites, profile, notifications) - the Share button prominently sits in the middle and is raised above the others. It also protrudes onto the body of the application... such that is is always visible and very clear as to what the application's focus is.

Also worth noting: Instagram has a very clever header. It is slightly transparent, contains the photographer's profile and image title, and it becomes persistent only when you scroll through a specific photograph and it's comments. Once you get to the next photo, the header changes. Great looking, unobtrusive and informative.

Sample picture from Instagram - just to give you a taste of the application. It is a picture from Lake Tahoe: