There's an interesting article about how some security researchers have managed to forge valid certificates issued by certificate authorities that had been signed with MD5. It's covered in more detail in Security Now podcast #177.

The problems affects a number of big name companies (inevitably, Microsoft is one of them) and some big name certificate authorities too.

I'm a bit surprised that the big companies would still be using MD5. It's been known for a long time that it is not that secure and other choices are available now.

MD5

MD5 Security blog