Navigon not only produces navigation software for the iPhone; their (probably) much bigger business is selling GPS navigation units for cars. These units can currently only be updated from Windows operating systems. However, an out-of-the-box Parallels installation running Windows may not be able to use the device.
My newly-bought Navigon 7310 mounts on the Mac as a removable drive, and contains some awful Mac software which basically only opens the manual using a browser. To update the device and the maps, you'll have to download Navigon Fresh from Navigon's web site -- sadly, a Windows-only app.
When I tried to run this via Windows in Parallels, it seems Parallels' SmartMount capability interferes with the Navigon USB device: if SmartMount is active, Windows only shows two drives with no data on them. Consequently, Navigon Fresh is not able to connect to the device.
After disabling SmartMount, I was able to install updates on my Navigon unit.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20091113060352240