Imagine an app screen used to fill out a form and then either Ok, or Cancel - an activity that on a desktop application might be a dialog box. The keyboard will be displayed, so we have limited screen space on a phone, and to ensure everything fits on all devices the screen content will be wrapped in a ScrollView, so it'll scroll if the screen is small.
In this app I use an ActionBar on all activities, to provide an easy and consistent way to move around the app.
Put Ok/Cancel at the top of the activity, under the ActionBar. If the user scrolls down, the buttons scroll off the top. I don't like this much as it means if the user goes down to enter the form data, they have to scroll BACK UP to get to the buttons. But what's good is the buttons are clearly visible right from the start - the user knows their options, though things appear in the wrong order (you need to fill the form out before sending it)
Have the Ok/Cancel buttons at the bottom of the form. The problem with this is they are not visible when the screen opens. New users may get confused and wonder what to do. I'm requiring they do some scrolling to even find the most important buttons on the screen. But the advantage is that the form to be filled is shows first, which is the first step they need to take, so in that respect it's logical.
Move the Ok/Cancel buttons into the ActionBar, so they are always visible. If I do this they will be much smaller. The approach would be much like the iPhone. So I'd be using the ActionBar to provide context-sensitive options for users to work with.
Remove the ActionBar from activities like this, and simply have Ok/Cancel buttons to select how to exit, and retain the ActionBar for the main screens. Though doing this may work on phones, it won't work so well on tablets.