For all of you who graciously helped me with suggestions, I thought I'd write back with the results of our problem with our new F720 and the AIT-2 (Sony SDX-510C) tape drive so that everyone could benefit from our experience.
The final justification from a NetApp engineer for why the AIT-2 drive would not work with our filer was due to insufficient cache on the F720's processor. Although we actually got a few complete backups using that tape drive, subsequent dumps would always fail until we got to the point (months later) that the filer itself would time out while talking to the tape drive at the beginning of a dump. We bought the filer February 2 and we got a DLT7000 tape drive on Monday, May 22, and now our problem is (supposedly) solved. We have had no problems with the DLT7000 so far (although it has less capacity -- and it's only been two days :-) ).
We never tried the AIT-2 tape drive with any third party backup software.
Thanks for your help!
Amy Caldwell Texas A&M Nuclear Engineering