Today I ruled that if you are tumbling through a hostile square to flank, you do are not actually flanking until the following round (due to the difficulty of maneuvering through without provoking attack of opportunity you do not have the timing, presence of mind, or the orientation of where your body is facing to take advantage of flanking). Whereas if you advanced from the other side to flank or were tumbling around an opponent then you'd get the flanking bonus on the same round. Was this too harsh? The PHB is not overly detailed in this sort of thing. 
