Brad, sorry for the inconvenience, but to the best of our knowledge this is a limitation on Visio's side of the equation. When we last checked, they didn't support arbitrary pattern fills. There are some specific ones that work, but OmniGraffle does have the ability to create fills that Visio can't handle.
We try to maintain as much visual fidelity as we can when exporting, but in cases where the file format we're going to lacks a feature that OmniGraffle has, we try to make a sensible substitution rather than limit what OmniGraffle itself can do. That's what's going on here.
If you're able to send the file to
omnigraffle@omnigroup.com we can take a look and see what can be done, though. Thanks for the help!