Well, the news is indeed shocking. Windows 2000/XP/2003 follow the same source code. Not sure about Longhorn, but since the source code of a "current" Operating System has leaked, the users will be have to be very careful.
Since hackers can now find exploits that MS hasn't found and there will be a lot of attacks initially. They will also be able to code viruses that are *impossible to remove or impossible to detect* -- since now they know how Windows work. The only alternative will be formatting the computer and reinstalling Windows. What if hackers code something to prevent that as well? Opportunities are endless if you have the source code of a program. That's why I personally do not like open source - it is dangerous.
Edit: It is good if it goes to good programmers. But this world is full of good and bad.