I'd have thought that 3rd Party Products would be the right forum...
Anyway.. the (maybe) helpful part of my reply: Omnigraffle Pro can work with Visio XML files (save option for Visio 2002 or newer). Not really a dedicated visio viewer and it costs (but not _that_ much).
As a bonus - as well as getting access to Visio diagrams, you'll be able to easily produce diagrams that are the envy of all visio users..!
p.s. One for the grammar pedants there
