Security researcher Mikko Hypponen reports finding a piece of malicious software that was cryptographically signed by a forged Adobe certificate originating with Government of Malaysia: Malaysian Agricultural Research and Development Institute, whose signing certificate was "stolen quite some time ago."
There are several hundred parties that are trusted by OSes, browsers and software to issue certificates, from Verisign to many national governments. A computer receiving a software update signed by a forged certificate will not be able to tell that there's anything funny about the update, but installing such an update could result in a thoroughly compromised computer.
I've been hearing persistent reports of this from security researcher friends, including reports of signed malware that can take over mobile phones and computers, compromising them so that their cameras and mics can be operated covertly, their keystrokes logged, their files plundered, etc. And the worst thing is, if you don't install updates, you can end up with security vulnerabilities that leave your computer liable to takeover by malware that does just the same thing.