Hmmm... I wonder about that... If we assume there WILL be DLC, and that it will be at least as large as The Pitt or Point Lookout, and that it will likely come out at a point when many players have maxed out their starting characters.... then a level increase might be required... Maybe up to 35, or 40 depending on how many and how large the DLC will be.
Honestly, with a game like Fallout and a system like SPECIAL, there comes a point when leveling should just stop, otherwise you can max everything out and become a god, and none of your specializations or character building decisions matter any more. In my opinion, whatever the level cap is in a game, it should be impossible to reach without putting in some serious, serious, effort and grinding in that direction. You shouldn't naturally hit the level cap just by doing all the quests. You should have to put in some time farming XP to hit the game limit.
My only hope/concern is that once a player hits level cap they are not God's like we were in Fo3 at level 30. Once I hit max level, all my specials were super high and skills all maxed out, so its not just the levels its the special and skill increases we get while leveling and the affect those have on gameplay. If I were worried, it would be about skills getting too-high once people hit level 30 - but I'm not worried about it one bit. I'm sure the balance will be very rewarding.
That is one thing I found interesting in FO2. You couldn't really hit 100% in all skills in single playthrough - the points just weren't there. But skills went up to 300 in FO2. The cost for each skill point over a hundred just kept getting higher. But it allowed you to "super-specialize" in your core skills if you so desired. You didn't have an instance like in FO3 where you have maxed your tag skills that fit your character story and now have to pick another skill to raise. Instead you could become a true master in your tag skills, pushing them beyond anyone else's in the game world if your were willing to totally fore-go any other skills to do so.