Microsoft released Game Room last week with a total of 30 retro games to choose from with content to be updated weekly. While it was thought that weekly would mean the very next week, a Microsoft representative has stated on their forums that games will start to be released weekly starting in late April.
If waiting a few more weeks to play games that have been available since the early 80s is too unbearable, Microsoft has some threads that you can post in to recommend new titles or suggest improvements for the service to kill some time. While most of the recommendations are bound to be for newer titles, an online version of Oink! would be pretty sweet.