IMO, this could be a killer app for android if they get the integration with existing phones right
I don't know what right is yet, but at first glance... things like:
Maps/Navigation as an overlay with the ability to push it back down and pull it back up quickly. Say you're working around a construction area, and you need a quick reminder of an alternate route. Or a traffic alert like an accident ahead...
Phone: Dial by name, accept VCards, crazy stuff like this.
Text messages: On screen with easy dismissal. Voice to text on your phone.
It's hard to say really because so few people have had these to know what might and might not work.