It seems customers of the Michigan-based Ally Bank will not have to wait too much longer for a Windows Phone app, as the bank has said on twitter that:
Our team is in the final stages of completing our Windows Phone app. It should be out in the next few weeks. Stay tuned!
The arrival of the app will be another example of the app gap (at least when it comes to essential apps) rapidly closing, and I suspect by the end of this year, as Joe Belfiore said, it will no longer be an major issue.