I really like the idea of an android app, the idea of posting out in public on facebook not so much..
If it was an app just for personal use, to go in and have a look through your own list of improved achievment updated in real time it would be a nice feature.
The facebook publishing part could be easily made a free choice by a settings menu anyway..
But having an alert on the cell phone for each achievment made would be a neat touch I think.
And just being able to go through the achievments list without using the clipboard menu, or even being logged into the game would be nice. Not a nescessity, just a nice little gizmo. I love gizmos..
Such an AH app could include other stuff of game info too, like score overviews, hours played, and so on and on..