...BUT I CAN'T DISCUSS IT OR MICROSOFT WILL MURDER MY FAMILY!
I was also in the XGD3 (AKA free copy of Halo Reach) dashboard beta back in the day. The irony in that update is that the new DLC doesn't work with the free disc, though Microsoft gave free Games on Demand codes to download the game via Xbox Live to make up for it, so you basically get two copies of the game.
Anyway, I can't give any details, and there aren't any details to give anyway. Hopefully they send me a free 3DTV so I can test out the rumored new features.