Seems clear to me: You can move DURING a full-attack action. WWA is a full attack(says so in the feat). So you can take a 5' step during the attack. However, it seems clear to me that the intent was NOT to allow you to use this to gain more targets as a result of your movement. Otherwise, one could conceivabley attack THIRTEEN opponents! Consider the following example:
XXX
XBXX
XXAX
_XXX
_ = space
X = monster
B = monster who dies pre-5' step
A = Whirlwind Attacking character.
A attacks, everyone around him, kills B and "during" his attack, he steps into the late B's space and attacks the rest. Silly. Silly I say!
My number one rule as a DM when judging this stuff: If it feels like abuse...it probably is.