It looks like October 22nd is the day you'll be able to buy Windows 7, but it was announced that it should release to manufacturing (RTM) in late July. Generally when a Microsoft RTMs, it is also released to their MSDN and other subscribers. The October 22nd date fits with the rumors we had heard earlier that had targeted October 23rd as the street date for the new windows operating system.
InternetNews has more coverage on the release in their announcement, It's a Date: Windows 7 Available on October 22.
eWeek reported that Windows was roughly a third of Microsoft's revenue in 2008. That equates to about $20 billion. I believe that Vista adoption has given Microsoft a rough time even thought Vista cleaned up its act with its first service pack. It is easy to speculate that this is a part of the reason that Microsoft had its first ever decrease in revenue earlier this year.
I've been using the pre-releases of Windows 7 for quite a while know. I also was an early user of Windows Vista due to the book I wrote on the topic. I can honestly say that it has been two completely different experiences. For the most part, the Windows 7 experience has been very good. Of course, I'm also now much more use to the interface changes!