Speaking to Shacknews, Bethesda's Pete Hines and Istvan Pely shed some light on the downloadable content plans for Fallout 3 (among other things). When asked about DLC, Hines noted that Bethesda was shooting for content that was "several hours." Hines elaborated, "Not just like a one-off thing, but something like where you can download it and play it for X number of hours," adding that it would be akin to Oblivion content like Knights of the Nine, "where it's like whole new quest lines, new stuff, that kind of thing." Reading between the lines, we're getting the message that Hines means not horse armor. While release plans remain in the dark, Hines does note that Bethesda is already "looking into" future DLC.
Hit the source link for the full interview, which contains talk about Fallout 3's design as well as how Bethesda feels about comparisons to Oblivion.