A public preview of Small Business Server 2008 is now available at the TechNet Evaluation Center. This preview does have a 180 day time-bomb and should not be used in a production environment.
You can download the SBS 2008 preview at http://technet.microsoft.com/en-us/evalcenter/cc184870.aspx
Get more information and leave feedback at the connect site https://connect.microsoft.com/sbs08?wa=wsignin1.0 and check out the Small Business Server 2008 Technical Library at http://technet.microsoft.com/en-us/library/cc527559.aspx.
One thing to note is that when you install SBS 2008 into a virtual environment, it will look for an internet connection. If you don't have the setup as recomended in the installation documentation, (meaning that you are not shutting off DHCP on your router) your network will not be fully configured as intended by SBS. If you are looking for a workaround, click here http://www.sbstraining.net/post/2008/05/Configure-SBS-2008-CITW-to-work-without-Internet-connectivity.aspx