the good news is: If you know Vista, you know the most about Windows 7. At least from a developer perspective. Several features burried inside of Vista are to be really used in Windows 7. So, no new driver model and stuff like this.
The good and bad news is: Well, there is some stuff new to come. Regarding the Taskbar etc.
I found the Windows 7 Developer Guide here http://msdn.microsoft.com/en-us/windows/dd206698.aspx which is for me the perfect entry as developer into Windows 7. Why? Because it lists the differences and what stays the way it was in a nice highlevel way. So it gives insides in what I have to expect and – this is the real good thing about it – it shows news that I might even not watched out for in the first place but might be of interest.
It has three chapters (Foundation, Experience and Web) worth reading. So grab a cup of coffee (or tea, what ever you prefer) and give yourself 15 minutes of reading.