Navimatics has the charts bundled into the app. They update it every few months and provide all new charts. All the charts are totally offline. You'll need a WiFi
connection to download the app because it's about 200 MB.
Getting the 3G model and never turning on the cellular plan is a good idea. That way you get the GPS
(which isn't present on the WiFi
only model). I have never turned on the cellular plan for my iPad
and have been using it for navigation
with all the different apps for almost a year. I was one of the crazy people on line the day the original iPad was released. And we're now docked 10 blocks from an Apple store and I'm holding myself back from being online on the 11th too!