The vignetting is not even in the upper and lower corners, so something's definitely very wrong. Did you by any chance have the lens hood mounted perhaps not quite fully? It's especially unusual that this is vignetting at 70mm - usually it's at the wide end.
I'd get all the stuff off the lens - filter, hood, etc. Then shoot some blank sky or wall - that will show you how much vignetting it has due to optics. Then add pieces back until you figure out where it came from. This lens does have fairly significant corner fall-off, although for some reason people don't complain about it like they do with the 70-200/f2.8 AFS VR-I - even though the amounts are fairly similar. However, despite that, the fall-off is even from corner to corner.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!