Back in October Microsoft showcased a pre-Beta version of Windows 7 at its Professional Developers Conference, and it just recently been confirmed that the full Beta should be available next month.
Those who register for and attend one of the upcoming MSDN developer conferences will get a DVD with Windows 7 Beta 1 on it, though since these start this month the handout will only come into force during some of the later events in January.
Event blogger Keith Combs stated that "My understanding is that the DVD will not be ready for the first few show dates but will be mailed to each attendee. We are targeting to have the DVDs in the materials from 1/13/2009 on."
It was around 16 months between the launch of Windows Vista Beta 1 and the official release, so if we're to believe that Windows 7 needs a similar timeframe you could expect it in early summer 2010. Probably best not to put any money on this just yet though.