"Based on internal testing and the beta 2 feedback around product performance, we are revising our development schedule to deliver the 2007 system release by the end of year 2006, with broad general availability in early 2007," a Microsoft representative said in an e-mail. "Feedback on quality and performance will ultimately determine the exact dates."
With increasing pressure from other software companies to produce things that not only run well, but are secure, it's imperative that Microsoft spends a lot of time in quality assurance. This is a better than the practice some companies have of releasing a product as fast as possible, assuming they can patch it later. Microsoft is already under scrutiny by many security companies, and with all the promises behind Vista and Office 2007 they will want to shine.