This is one of the differences between Poker Stars
and Full Tilt Poker
On Poker Stars they do instant balancing. It happens frequently that a player is moved to balance the tables and then seconds later the tables need to be balanced again and the vacated seat is filled.
On Full Tilt they freeze the tables when balancing is needed (at least later in tournaments) and wait until all hands are finished and then balance the tables. Sometimes the need to balance is eliminate by further player elimination.