ABFer - first you need to be clear about what you want to do with it. The iPad has "assisted" GPS, which means it gives you a simulation of GPS, based on the triangulation from the cell phone towers in range of your location. You will have to activate the cellular antenna with a carrier to get location information from it, and it's not going to be as accurate as a true GPS. Translation - I wouldn't recommend using it for navigation of an airplane. As a backup, or for entertainment purposes (and if you look out the window now, you can see blah blah lake to the east), maybe...but don't bet any lives on it.
That said, your best course of action would be to get it equipped for whatever carrier you have a cell phone with already. Two reasons behind that. First, most of us have a cell carrier that gives us good reception where we are, but won't necessarily work everywhere in general. i.e. I used to have a phone through Sprint that worked great at home, but when I went to visit family, the phone barely functioned. My family lives on an island, and their house is just barely in range of the Sprint towers on the mainland, so the phone would constantly flip between Sprint and roaming. Basically, I never had a consistent enough signal to make/receive a call, and due to the constant network searching the battery was always dead. Not a knock on Sprint...just poor reception in that particular location.
The second reason is pricing. I'm currently with Verizon. If I were to buy an iPad, I'd be able to add it to a Share Everything plan and activate service for $10/month. Then the iPad and my current cell phone would share their data usage (2 gig plan, phone uses 500MB in a month, that leaves 1.5GB during that billing cycle for the tablet without paying overages). Whomever your carrier is probably has a bundle deal.
Before you spend any money, take a trip in the plane and make sure that you actually get cell reception at the altitude where you want the iPad to work.
The last bit I'll contribute (unsolicited) is to be sure you want an actual iPad, not just a tablet in general. I'm partial to Apple products, so I'm not trying to throw them under the bus. But you need to realize that all Apple mobile devices must be activated/registered/partnered to a computer. Once they're activated, you can do almost everything wirelessly, but ultimately you'll need to ALSO own a computer (Win or Mac doesn't matter).
