There's a whole host of information out there for people looking to upgrade their AD environments to Windows 2008 from Windows 2003 or even 2000. Rather than do the hard work of pulling this guidance together myself, I'd like to point out the following Technet blog posting as a great place to start. Courtesy of Joey Snow:

http://blogs.technet.com/jsnow/archive/2008/03/27/upgrading-active-directory-to-windows-server-2008.aspx  [RH]