A source close to Microsoft has confirmed to me that Windows Vista will in fact carry the build number 6.0.6000. He found this out by receiving a mail with the build stamp "Microsoft Windows Mail 6.0.6000.16384" so that ends that speculation.
Also the MSDN subscription page has been updated with the following information
Windows Vista and Office 2007: Availability on MSDN Subscriber DownloadsLooks like we'll be seeing this appear soon.
Windows Vista and Office 2007 will be available through MSDN Subscriber downloads within 7 days of release to manufacture (RTM). MSDN Subscriptions is committed to making new content available to our subscribers as soon as is practical.
Update: Several key Microsoft employees has firmly stated that Vista has not been released to manufacturing. The day might be soon, but is certainly not today, or tomorrow. There's still a bit more work to do. Build 6000.16384 is real, but it also doesn't mean RTM. These days, Microsoft doesn't have to increase the major version number when building, so theoretically, any sub-set of build 6000 (6000.16385, 16386, etc...) can be RTM.