A last-minute bug discovered in Vista has delayed its release. Originally intended to ship to PC manufacturers in bulk two days ago, Vista has been pushed out a few more weeks to November 8th. The bug was fairly serious, one that could corrupt a system to the point where a reinstall would be required:

Allen said the Vista team discovered the bug, which "would totally crash the system, requiring a complete reinstall," in Vista Build 5824 on Oct. 13. The team fixed the bug a week later in Vista Build 5840, he said, but the delivery of the operating system to PC makers was delayed.
A flaw that serious in nature could put a definite hamper on shops looking to get machines out the door as fast as possible. Since late November and through December are prime sales times on machines, any further delays would definitely cause problems for both the shops and Microsoft.