A little bit earlier today, Microsoft gave a press release about their events planned for Comic Con in LA. As part of this release, they also mentioned when the dashboard update will become available, August 11. By the sounds of it, all features of the update will go online on that date, including the Games on Demand.
The following games were specifically mentioned in the release for the Games on Demand:
-Lego Star Wars: TCS
-Sonic the Hedgehog
This press release also had 2 more things of note to those not attending the con. First, on August 25, The Guild will be returning with the beginning of season 3, as well as a new Xbox Live exclusive show, Girls Go Geek. The other major thing to note is that this release specifically says that the update containing Facebook, Twitter, last.fm, and Zune HD 1080p streaming will arrive later this fall. So much for the "1 update a year" plan they had going on... not that I'm complaining.