Thanks for the kind words, blewis! Duping my response to the blog comment here: for the time being, OmniGraffle for iPad doesn't have support for importing or exporting Visio files at all. Due to the time constraints, it simply didn't make the cut for the initial launch.
We do want to add the ability to import Visio documents at the very least. We have a feature request for this in the tracking database - email the
support ninjas, and they can vote that request up on your behalf
Support for exporting to the Visio format is going to be contingent on how efficiently we can import Visio files, but if we can make it work, we'd like to get it in there.