According to reports, it looks like Windows 8 may be arriving as soon as April 2012. The rumor came from a trusted source that has confirmed the soon release.
The same source also states that there will be no pre-beta build, but instead just a simple beta build. This data makes a lot of sense considering that the next Eee Pad Transformer is set to release by summer 2012 and is supposed to run Windows 8. This would give Windows 8 a month or two of shelf life before being installed on new Windows 8 tablets.