Microsoft have made SBS 2008 RC1 available via Public Preview. It can be downloaded from here.
Other useful links from the TechNet site:
Very handy documents to download and read:
Three very useful resources to keep an eye on while evaluating and testing SBS 2008:
And finally, if you plan on testing this in a Virtual Machine, check out:
Note that SBS 2008 requires 4GB of RAM. It won’t install with any less memory than this. You can use an evaluation of VMWare Workstation or either VMWare Server or VMWare Server 2.0 Release Candidate. Both VMWare Workstation and VMWare Server 2.0 Release Candidate support USB 2.0 devices, so you can test server backups/recovery with removable USB drives with these products.