Read [link|http://www.ftp.cl.cam.ac.uk/ftp/users/rja14/toulousebook.pdf|http://www.ftp.cl.ca.../toulousebook.pdf] for an analysis of why open and closed systems are (to first order effect) equivalent when it comes to security. Read it carefully and note that while bugs are found and fixed in similar rates in both, the ease of finding bugs with source available means that the open system winds up being of higher overall quality.

To the extent that Microsoft makes the transition from being closed to open, its overall quality will improve. Unfortunately the transition is accompanied by people finding lots and lots of bugs, many of them readily exploitable.

If Microsoft code was open all along, its new-found openness would be a non-issue. But it isn't, it is crap internally, and the transition could be painful.

Cheers,
Ben