For whatever reason, under GM Ted Thompson, the Green Bay Packers haven't made a big effort to stock their safety position with depth. When S Atari Bigby struggled with injuries in 2008 and 2009, his absence exposed their lack of depth. It was so bad in 2008 that they switched Charles Woodson to safety for one game, and in 2009, they even gave LB Brandon Chillar some reps at the position.
Actually, at the start of the 2011 season, the Packers had as much depth at safety as they've had in recent years. Nick Collins hadn't missed a game since 2007 and Morgan Burnett had returned from an ACL injury. Charlie Peprah was held in reserve as the top backup after starting in place of Burnett for most of the 2010 season. Instead the position was blown open by the (probably) career ending neck injury to Collins and the collapse of Peprah, who was bad throughout the season, before getting embarrassed during the playoff loss to the Giants.
The Packers haven't uncovered every stone to upgrade at safety, but selecting Jerron McMillan in the 4th round and signing two undrafted rookies is a big haul for them. Obviously the release of Collins has made this more of a priority than it had been in recent years, but the 2012 draft class was weak at safety so it's understandable that they went with some high risk/high reward prospects, especially when they needed to use their high draft choices to bolster the pass rush and defensive line.
JSOnline.com takes a closer look at M.D. Jennings, who the Packers saw something in last year after signing him as an undrafted free agent. I'm not a believer in him yet, but I expect he'll get a much longer look this preseason with a full offseason to prepare, and with the expectation that he should be the top backup, or even the starter alongside Burnett.
He's probably a long-shot to make the roster, but they also signed Micah Pellerin, an undrafted rookie out of Hampton, to give them another option. According to NFL Draft Scout, he's undersized, but showed good coverage skills at the East-West Shrine Game. Right now they need as many options as they can get.