Turning verify checksums & ticking on ignore invalid checksums (ie: don't verify images) can sometimes override this issue. But then again there are corrupt dmgs out there. If you have Toast, sometimes it's worth trying dmg's on it. Also drag & drop the dmg onto disk copy rather than double-click it. Somebody recently solved a similar issue by repairing permissions.

See these links for xtra help:

Thread, & Apple article , & Apple forum. Hope these lead to a solution for you.