One of my complaints about the (not very good) support for system state backups in WS08 is that you can't do a system state backup to a critical system volume. A critical system volume on a DC includes, IIRC, the boot volume (the one with the boot loader), the OS volume (the one with \WINNT), the volume with the AD DIT files, the volume with the AD log files, and the volume with the SYSVOL share.
There are a lot of domain controllers that do not have a volume that isn't one of these five, so therefore they can't do a system state backup. And no, they can't do a system state backup to a file share either; that's another gripe I have about SSB on WS08.
I mentioned this at the MVP Summit this week, and I got two responses. From the current backup PM, best practice is not to do a system state backup, but to do a critical volume backup using WSB. This leverages VSS so you get the time savings of VSS and the space savings of block-level differential backups. The problem of course is that you are backing up the entire volume, not just the files you need for a system state restore. And for the backup target, you still need a spare local volume that isn't one of the five I mentioned in the opening paragraph. Not very helpful.
The other response I got from the outgoing backup PM. He pointed me to KB 944530. It describes a registry key you can set that will allow WSB to perform a system state backup to a critical system volume.
That will be helpful.