Anticipating the launch of the Mothership Zeta DLC for Fallout 3 on the Xbox 360 and PC, MTV Multiplayer asked Jeff Gardiner of Bethesda via an email interview if it would be possible to raise the level cap from the current 30. His answer: it's not advisable as it could make the game unbalanced.
The existing level cap of 30 was set forth by the Broken Steel DLC. Explained Gardiner, "I know from the outside this seems like a fairly simple proposition, but doing this could unbalance the game in a variety of ways."
He did not go on to further elucidate these "variety of ways", but he did say that Fallout 3 (also available on PS3) was intended from the start to be replayed several times anyway, from a variety of different character types. Therefore, if you'd like to enjoy the game more, you can always start from scratch. But don't worry, he did not entirely close the door on possible increases in level for the future:
We're very happy that people enjoy Fallout 3 so much that they want to keep playing it, and I'm sure we'll look into addressing these concern in future games.
At least it wasn't an outright "no", right?