More Xbox Originals Information
Major Nelson has answered the question about achievements and Xbox.com now has a page dedicated to Xbox Originals:
I have gotten quite a few questions about yesterday’s Xbox Originals announcement. Specifically, folks want to know if these games will have additional functionality like Achievements and Gamer score. The answer is no. These are the original games that were created before Xbox 360. In order to preserve the integrity of the original gaming experience they provide, they will have the features available at the time of their initial release. I hope that clears that up.
I would assume the Guide (along with Live features like the friends list) will still be available while playing these downloadable games though, if not, they should be.