[denteddagger]

I think your right Whewt about Aizen using Urahara to further his ambitions for greater power. What may be his downfall is giving Urahara insight on his Zanpakatous abilities and the time to find a way to counter them.
As mentioned in many posts Aizen is powerful but how much of his power is dependant on people being under his total hypnosis. We can only wait and see what surprises Urahara has for Aizen.