If using the built in Maps application (Google Maps), you are still required to have a data connection (3G or wifi) because the maps are not stored locally and need to load. While the device GPS can track your location, you wont have the maps without a data connection.
You can use applications which do store maps locally as an alternative, but there isn't many for iPad just yet as someone mentioned. If you have one of the iphone ones, they work as well.
One great one that I use for marine navigation and boating is iNavX, makes great use of screen real estate and GPS works even with no service