According to a listing on Amazon.com's U.K. site, the first two pieces of downloadable content for Fallout 3 have been bundled and will be available on May 29th. The site lists Xbox and PC versions.
The DLC (well, technically "expansion, since you won't be downloading it) is listed for £14.99. That's nearly 22 American dollars. Whether the add-on bundle will be available here in the States or that price will apply remains to be seen, but if you're European, you can pre-order now.
This seems like an odd choice: If you're a big enough Fallout 3 fan to want the expansions, you probably already have them. Although this does serve the interest of people who either can't or won't connect their Xbox or PC to the internet.