I believe MathOverflow - a formerly non-SE site - targets graduate-level questions, while math.SE has a broader target user base.

Yeah, MathOverflow was a SE 1.0 site (when we had white-label Stack Exchange networks rather than internally made ones) which has since been added into the real network with the SE 2.0 rollout. Math.SE and MathOverflow seem to function well together even they though have similar areas of interests.

Could you explain the expression, "white-label?" First time I've heard it.

The Wikipedia article explains the concept, but basically the initial business plan for Stack Exchange was to sell the software to companies / groups that wanted to run their own system versus the all in-house management we have today.

This gives a decent explanation: http://en.wikipedia.org/wiki/White-label_product