Bull rush says that the character shifts into the vacated space.
Is this shift optional? (It became highly important when there was a Wall of Fire last night!)
To clarify: all non-forced movement is optional. Only when the power text explicitly requires that you move the full distance (and I couldn't find any instances of the term "must") Thus, any movement that is not forced movement is optional.
As per Move in the glossary:
Inasmuch as bull rush is a push 1 for the enemy, and then you shift into their square, it seems to fall squarely under the move rules, and thus isn't required. There are no feats that contradict that assumption. Should I contact cust serv for you? (Replies tend to be spotty and inconsistent, but if your GM isn't happy with the move ruling...)