As you may know, Midway filed for bankruptcy last February, and the majority of their assets and studios have since been bought out by the likes of Warner Bros.and THQ. As such, the Midway games that had been released via the Xbox Live Marketplace seem to have been pulled and are no longer available for purchase. Those games are as follows:
If you have already purchased these games, you will still be able to play them and redownload by viewing your Purchase History. The game's will also still retain the same LIVE functionality as before, nothing should change there.
For you lucky Americans, most of the above games are still available for purchase as download codes from Amazon.com, though I'm not 100% sure whether the codes will still work. I don't see why not, but there's always that chance that they can no longer be redeemed. EDIT: According to http://www.trueachievements.com/D+Fineout.htm
's research, these Amazon codes unfortunately no longer work.
To put my speculation hat on for a minute, it could be possible that some of the above games will be rereleased as part of the Xbox Live Game Room but under, for example, WB's name so that they received more of the money from sales of the games. There's no evidence to suggest that theory, but most of those games seem to fit the bill of what we're to expect of the Game Room releases, so it's surely a possibility?