This brings me to my topic and the question of what XBLA titles are being held back as fill-ins in case of emergency? These games (all of which appear to be classic Arcade titles) are sitting there, complete and certified waiting to be put on XBLA. Well, the answer IMO lies in a little research involving the Xbox.com game catalog and the ESRB website. Xbox.com tends to place the game page for XBLA titles out there when they are ready and may go live, and the ESRB page rates games and lists them in the order they were rated, which many cases is several months ago.
When I first started researching this, it was quite amazing to see a pattern jump right out of the ESRB website involving the so called "Classic" arcade titles. You could see ratings being posted for titles (in relation to retail games), yet not being released unless it was part of a "twofer", usually along with an original XBLA title. Examples were Yie Ar Kung Fu, Super Contra, Golden Axe, Xevious and Double Dragon. In each case, it was rated far in advance of when it actually hit XBLA, and was doubled up with another game.
Based on my research, I have concluded that Tron, Fatal Inertia and Fatal Fury Special are more than likely the next three classic arcade titles (not necessarily in that order) due out. Followed closely by Discs of Tron, Streets of Rage 2 and Tempest. All of these have been rated by the ESRB since at least mid June, yet haven't even sniffed XBLA yet.
So anytime you see two titles being released, and one is a classic arcade title, more than likely you can assume there are another five classic arcade titles safely tucked away, waiting to be sprung onto XBLA on a rainy Wednesday morning.