The quick answer seems to be "not much". Last week Lukas Grunwald showed that the highly touted RFID chip in the new electronic passports is not as secure as its proponents want us to believe.
Also last week it appears that a security flaw was found in the (closed source) wireless drivers for a certain class of Intel wireless chips, both under Windows and Mac OS X. To the best of my knowledge no similar exploit seems to exist for the open source drivers to the same hardware that are used under Linux.
So does open source create higher security? I don't really think so. I believe the chance of better code review by well meaning experts is evened out by the fact that not-so-well-meaning people have the same level of access.
But what I think this once again clearly shows - closing your sources doesn't make your system any safer. Not that the recent debacles with voting machines or the ongoing nightmare of Windows security issues didn't provide enough examples for this in the past. Still, it seems that this idea is hard to defeat - I keep hearing these same arguments over and over again - "if we open the source we expose ourselves to so many more security threads". If you think that, please think again.