From what I gather, the actual RTM date has slipped so close to the holidays (November 2006 from somewhere between August and October) that many OEMs and retailers just said "Fuck it. We don't have time to set up our marketing, supply chains and salesdroids for this and don't trust you'll get it out in time anyways, so just punt the big hype launch till January 07 when we have more time".
So it will still "ship" in 2006 and Volume License customers will still get it (like they'll roll it out any time before 2008 anyways), and consumers will have to wait till 2007. Maybe the OEMs will do free upgrade vouchers for xmas PC sales.
As for what is causing the slippage itself, there are several interesting posts to [link|http://minimsft.blogspot.com/2006/03/vista-2007-fire-leadership-now.html#comments|this thread] at mini-microsoft that point to a burdensome development process implemented after the "reset" last year as well as other management problems.