From John Porcaro (Microsoft's Group Manager in Global Games Marketing) on the GamerScore Blog:

We just posted a Backwards Compatibility update, with 39 new titles and 9 updated titles--the largest number of new games updated since the original list of over 200.
It's interesting to note that this is the first time the list includes titles that are not on the market yet for Xbox, with Spy Hunter: Nowhere to Run, The Legend of Spyro: A New Beginning, and Lego star wars II. And of these, Lego star wars II also has an Xbox 360 SKU - another first for the BC list.
The list includes Burnout 3: Takedown, Counter Strike, Dead to Rights, ESPN Major League Baseball, Tom Clancy´s Ghost Recon - Island Thunder, Lego star wars II, Mortal Kombat: Deception, Over the Hedge, Rogue Trooper, Serious Sam, and Spy Hunter: Nowhere to Run. It also includes updates to Half Life 2, The Incredibles: Rise of the Underminer, and several more.

You can play cross-platform between Xbox 360 and the original Xbox over Xbox Live. And with anti-aliasing, these original Xbox games will look better than ever on Xbox 360 running in 720p and 1080i.
The team is continuing to work hard to certify as many original Xbox games to work on Xbox 360 as possible, and we'll be adding to the list. Check Xbox.com/games to see the current list and information on how to get the free software update.

News-Source: gamerscoreblog.com