Late last year a roadmap shown at PDC 2009 revealed Windows 8 would see a "~2012" release. While that little tilde before 2012 had us believing it would be more like the first half of 2013 and Microsoft was just affording itself some wiggle-room, it looks like we'll see the OS sooner than 2012.
A former Microsoft employee by the name of Chris Green recently posted a visual graph detailing Microsoft's product lifecycle index. As msftkitchen points out, this graph contains an awful lot of information that can already be found from Microsoft's Support Lifecycle Index (opens in new tab). However, Green has mentioned some things that aren't listed on the MS Lifecycle Index; namely dates for Windows 8, Windows 8 Server and Office 15.
Green's graph have Windows 8 RTM down for July 1, 2011, with mainstream support from July 1, 2011 to July 1, 2016 and extended support from 2016 to July, 2021.
Office 15 is pegged for a July 2, 2012 RTM; mainstream support from 2012 to June 30, 2017 and extended support from July 3, 2017 to July 5, 2022.
Green has Microsoft Server 8 down for the same dates. So that's a July 2, 2012 RTM with mainstream support from 2012 to June 30, 2017 and extended support from July 3, 2017 to July 5, 2022.
Green left Microsoft on January 12, at least, that's when he blogged about his leaving Microsoft. However, the update that has everyone talking was posted on December 9, presumably while he was still a Redmond employee.
You can view his roadmap here (PDF warning).