The Windows Home Server beta continues to progress: build 06.00.1371 was just released for beta participants. If you missed out on the earlier beta sign-ups, but would like to participate, there’s a survey to fill out right here. With full backups of all computers on your home network, WHS is a compelling product for many homes, but even more so for mobile computing users. Having full access to your home files on the road or remote controlling a home PC while away is the "killer feature" of WHS in my opinion. Yes, there are plenty of on-line backup services and remote clients, but I expect the all-in-one native nature of WHS to be compelling.
Colin Walker has chronicled his testing efforts and opinions right here; if you’re at all interested in WHS, I highly recommend reading his thoughts.