Don Murphy in the SBS2k list said..
Quite a few months ago I ran into an issue with ShadowProtect on SBS 2008. Long and the short of it is there is an issue in the Exchange vss writer that was fixed in Update Rollup 9.
This should help a lot of people with there backup woes on SBS2008.
It appears as DPM issue but Shadowprotect and other applications that use the vss writer experienced issues as well.