iOS tab bar for action buttons instead of mode?

The docs I've read seem to indicate the intended purpose of the tab bar at the bottom of an application window is to provide access to different modes; the Clock app is a good example of this. On the other hand, we have applications like Mail which seem to use the bottom bar for actions (like creating a new message). I have an app which would similarly benefit from using the tab bar at the bottom for actions, so I am wondering -- is this OK from the standpoint of the common user interface guidelines? Am I ruining some iPhone goodness if I design my app in this way?

Excellent, many thanks!