Update 4:46 EDT: The WSJ has a larger AP story here. Not much info, just the typical wanting to spend more time with wife/kids argument, but it does say:
Authorities have said that Mr. Rattner, an investment banker, was unlikely to face charges in the investigation, which involved a giant state pension fund that provides retirement benefits for more than one million government employees.
Given the abrupt resignation, it certainly seems like something's up. The most likely culprit is the pension scandal, but I'm hoping for something more interesting.
Update: 4:55 EDT: Better story from Politico here. Look's like we know all we're going to for awhile, but if something breaks, I'll update this post with links.
(h/t Ben Smith)