You might take a look at TouchDraw. It can interact with MS Visio and is fairly capable for business diagrams, maps, floor plans or network diagrams. I haven't used it extensively, but it's handled everything I've needed so far.
Although its more expensive than the other options, I'm a big fan of OmniGraffle. I've used the mac version for years and as far as I can tell the iPad has the full set of features present on the desktop version.