What happens when the space behind someone is already occupied by a wall? Do they take damage, fall prone, does it not work etc.?
It counts as a successful Bull Rush for the purposes of any abilities that care about things like that, but they do not move.
This feat from Complete Warrior, most famous for its ability to allow you to apply Power Attack penalties to your AC instead of Attack during a charge, also allows you to move foes that you Bull Rush to the side, as well as forward. So if they’re against a wall, this could allow you to move them along the wall.
It also allows you to Bull Rush one enemy into another, knocking both Prone.
The Dungeoncrasher ACF from Dungeonscape allows a Fighter to do sizeable damage (8d6+3×Str) with this maneuver. This is so reliable and potent that this ACF alone is enough to push the Fighter up a Tier.