But you can trust them with your medical records...

According to a Wall Street Journal report, the breach was indeed discovered in April. But according to sources who spoke to the WSJ’s Damian Paletta and Siobhan Hughes, it was in fact discovered during a sales demonstration of a network forensics software package called CyFIR by its developer, CyTech Services. “CyTech, trying to show OPM how its cybersecurity product worked, ran a diagnostics study on OPM’s network and discovered malware was embedded on the network,” Paletta and Hughes reported.
Got that?  Reason this was found was "Let us run this to demonstrate... oh my... did you know about this?"

The massive hack into federal systems announced last week was far deeper and potentially more problematic than publicly acknowledged, with hackers believed to be from China moving through government databases undetected for more than a year, sources briefed on the matter told ABC News.
“If [only] they knew the full extent of it,” one U.S. official said about those affected by the intrusion into the Office of Personnel Management’s information systems.

Anarchangel put it this way:
At this point, you have to assume that this is the worst case scenario... it looks very much like it is, and they can't ensure otherwise. 

I'm assuming all records, reports, and supporting materials for clearance investigations, for military purposes, DOD, DOE, state, and other national security and intellegence apparatus, conducted outside of military channels... which is most of them... Including all clearance investigation for service attached to national security and intelligence apparatus, and associated background briefs, and all military records and reports obtained and attached to those reports records and materials... are compromised.
Which means the Chinese have had a year to dig around and copy and study.. can you say 'blackmail material'?  Can you say 'intelligence breach of unbelievable proportions'?

Makes you wonder if this has anything with Obama & Co. wanting to badly to get this fast-track trade bill through.

And:
Federal investigators are trying to determine whether the massive hack into federal systems announced this past week impacted far more than the estimated 4 million current and former government employees already acknowledged by the Obama administration, sources familiar with the matter told ABC News.
In particular, investigators are considering the possibility that private citizens who never worked for the U.S. government may have also had personal information compromised, sources said.
At the heart of concern are forms filled out by federal employees seeking security clearances. The forms -- known as SF-86's and used for background investigations -- were exposed after hackers infiltrated the Office of Personnel Management's information systems in December, according to the sources.
Acting as the government's human resources division, OPM conducts about 90 percent of background investigations for the federal government. And federal employees who submit the SF-86 forms provide personal information not only about themselves but also relatives, friends, and potentially even college roommates.