Check out Diagrammix, much cheaper the OmniGraffle....
I was about to dismiss your comment, but after digging around, I found that they do have some nice networking templates.
And $20 USD is a nice price.
Did you list this product merely as an alternative? Or are you using it as a network diagramming product that you enjoy using?
A few years back when I was spending much more time wearing a networking hat, I had explored and purchased a copy of Omnigrapple for network maps.
Its a good product from a solid company.
It will probably never have all the different (supplied and user created) network image templates that Visio has.
That;s why I personally chose Omnigraffle - the power, flexibility and ability to access visio documents. I can't knock the OP for going with the other product, 20 bucks is hard to beat especially if you need it for a one thing.OmniGraffle can import many Visio stencils allowing you to use them in OG.