I found that interesting. After such a long feature on DX10 and the marketing hype that came with it prior to Sweeney's interview, he states very simply that Vista and DX10 are probably not going to be worth upgrading to for 6 years.
Well, that's not quite what he says. I think when he says "we ought not even consider it" is a reference to making games that require DX10/Vista. He says that they should not be making games that require it until around then. He also says earlier that that many games will be shipping with support for both DX9 and 10 from late 2007.
With that in mind, Vista vould still be "worthwhile", as you say, once the games start rolling out that use it, regardless of whether support for XP has been ditched at that point. This will happen long before 2012.