Did you try setting textAlign="center" ?
<s:Label text="My View" textAlign="center" width="100%" />
That works, I wanted to center the title to the center of the ActionBar, not the titleGroup.
I was able to achieve it using your strategy by setting the Labels width to (appWidth - (navigationGroup.width*2)) and textAlign="center".
To be honest, not sure if it is a good idea as it may cause issues with truncated text, overlapping navigation/action buttons. Trying to match the UI design... and it has centered title... I will test more...
When I do add my own label as to the titleContent I lose my styling. I have been avoiding setting font sizes and such as I want to leverage default font sizes specified in the "mobileTheme".
Do you know how I can access these "mobileTheme" various font sizes so I can match custom components with ones from Flex SDK?
I took a look under the hood and it appears to center the title property of the actionBar you are going to need to make a custom ViewNavigator and then add some custom skinparts inside of that. It sounds like a lot but it's not really. I'd help you out more if I wasn't so busy with my own work.
hope that helps you out.
titleAlign : center;