A friend was speculating about the target register outage being a fry run for a larger attack of more realtors. Here is my answer:
Anyone with the time, power and access to effect a nationwide shutdown of Target registers would be better off keeping it quiet and profiting off of the information they have access to. They wouldn't announce to the world they had control by shutting things down.
If there's a vulnerability you don't do a test run this large because it will get patched now and the fix that will get spread to kmart, walmart, etc.
It also occurs to me if the goal is to destabilize the economy you don't do this on June 15, you do it on Black Friday.