I would tend to lean towards the end of the year if not early 2010 due to the fact that the anytime upgrade offer for 7 like was seen with Vista won't be seen until 7/1/09. The second public beta if there's going to be one still has to be seen with the first 7000 build expiring on 8/1/09 a month later. Offset that by 3-4 months for any final product release.
When you start seeing MS information posted at the 7 home page then you will have more to go by there. http://www.microsoft.com/windows/windows-7/
I'm sure everybody by now has had a good look at that page.