All games are Required to support 720p, Developers can choose to support no other resolution but 720p, the consoles hardware can up or down-scale that to work with a 1080i set or a 480i set depending on your preferences.
720p is a widescreen only format, so if you down-scale it to 480i you will have black bars...
MS has said that they will give you options for how to deal with that, like stretch, and zoom etc.
I personally think any developer who chooses to go strictly 720p with no native 4:3 480i support would be foolish...
But yes it is possible that some games will have black bars, it depends entirely on the decisions of the developer.