It looks like the Canon DPP software was updated 10/18/2009:
http://www.usa.canon.com/consumer/controller?act=ModelInfoAct&tabact=DownloadDetailTabAct&fcategoryid=314&modelid=16303
You might also see if the images open properly using the built in OS X Image Capture app.
I know the Canon 5D Mk II RAW files will open successfully under Snow Leopard (fresh install) - perhaps some corruption came across when you did the upgrade install of SL? If you reinstall Snow Leopard over itself, it automatically does an archive install, which may also resolve your issue.
Good Luck!