For now there has not been a specific date announced as to when this will be available, but the features to be found will help to streamline your phone-based Market experience. Purchasing books, buying and renting movies, apps, and games that all stream to your phone are just some of the goodies that are going to be available. Books that are purchased will sync across devices based on your Google account.
The Market is also getting a makeover, which will hopefully make it faster and easier to discover great apps in ways the current Market can not deliver.
So far Google is hinting that this will be a very standard update and that all users running Android 2.2 or higher will get this without any special work needed.
Of course if you can't wait, you can also just download the .apk and enjoy the new Market now. A word of caution if you take this route: Rooted devices cannot rent movies from the Market; you'll pay for them but you won't be able to watch them.
[Google Mobile Blog via Droid Matters | Redmond Pie (thanks, Will!)]