Bethesda Softworks - Interview @ spong
Pete Hines fronts an interview with SPOnG ranges over a variety of topics, although Pete avoids anything interesting with no comment on further FO3 DLC plans or Obsidian's New Vegas. So, here's the only informative bit:
SPOnG: Will there be enough quest experience to be had in Broken Steel for players to level up to 30 without having to grind on pre-DLC quests/enemies?
Pete Hines: No, probably not, and that wasn't the intention. The intention was to remove the level cap so that if you have Broken Steel, regardless of whether you want to play that quest or not, or you want to start a new game from scratch, you can continue playing beyond level 20. Again, fans wanted to be able to take their characters to higher levels, so we included it. But it is not specific to the length of Broken Steel, at all. It is a very long climb to get from 20 to 30 and you'll need to do a LOT to get there.