There are two ways to reserve your iPad for the April 3, 2010 launch date, you can pre-order it and have it delivered to your US address, or you can reserve one for in-store pick up at one of Apple's US retail store. In either case, Apple limits the iPad pre-order to 2 units per customer. I guess this limitation is to warn off grey market sellers, ensure enough early adopters get their iPad's, and less likely is the rumor that supplies of iPad will be low on launch day April 3rd (in USA).
There are two specific lines of iPad's, one with WiFi only connection and another with both WiFi and 3G connections. Which one you buy really depends on how you plan to use the iPad. The great thing about the iPad is that its use will depend on the user and the apps s/he choose to install onto the iPad. People have to stop slotting the iPad into any existing electronic device categories. It is definitely not a "tablet computer" in the traditional definition or a "notebook" or "netbook" computer, so comparing it to any of these are like comparing apples (sorry for the pun) and oranges.
For me I believe the WiFi+3G version is the only viable line for this sort of device. My reason can be explained by how I use my iPhone. Many of the apps in my iPhone depend on readily available connection to the Internet. In Hong Kong (where I currently live) WiFi hotspots are not always available even though Hong Kong has the second highest penetration of WiFi hotspots in the world. What I mean by "not always available" is that one may not be able to get an IP address right away or the WiFi signal is too weak. When a WiFi hotspot is available, it usually requires the user to go through a tedious login screen before the user is allow online.
When I perform a task on the iPad, I do not want to think about the network connection or my login credentials. I will want to jump right to the task without having to worry about whether I can get a WiFi signal, if there is a WiFi hotspot that I have access to, wait for the WiFi hotspot login page to come up, or enter the require credentials. All these are interruptions in the workflow of accomplishing whatever task I want on my iPad. This is the reason I have my WiFi turned off on my iPhone 3GS, leave my 3G radio enabled and have an unlimited "local" data tariff plan from my carrier, Smartone-Vodafone.
With the acceptance of pre-orders Apple also reviews more information about the iPad and the custom apps that Apple had made (ie. iBooks, etc.), in particular the WiFi+3g version of the iPad. Apple states:
No-contract 3G service In the United States, 3G service is available through a breakthrough deal with AT&T. You choose the amount of data per month you want to buy — 250MB or unlimited. If you choose the 250MB plan, you’ll receive onscreen messages as you get close to your monthly data limit so you can decide whether to turn off 3G or upgrade to the unlimited plan. Best of all, there’s no contract. So if you have a business trip or vacation approaching, just sign up for the month you’ll be traveling and cancel when you get back. You don’t need to visit a store to get 3G service. You can sign up, check your data usage, manage your account, or cancel your service — all from your iPad.
So apparently the WiFi+3G version of the iPad will tie very close to the carrier partner Apple has for the country, in this case USA and AT&T. Or Apple has an API where it can allow the mobile carrier to publish and process usage data. From my past experience determining iPhone usage data from the mobile carrier prior to bill date is close to impossible. So the iPad's ability to show number of MB remaining in a fixed data tariff plan is something new.
Also the ability for iPad users to sign up and cancel data tariff plans directly from the iPad, means that the GSM micro-SIM within the iPad may not be removable or provided by the mobile carrier like past GSM mobile phones around the world.
Therefore, if you want the WiFi+3G versions of the iPad and you do not live in a country where the iPad is officially offered by Apple. I will suggest you wait until a few days after users in the "luck" countries receive their WiFi+3G iPad before purchasing your iPad from those countries. Given the nature of GSM mobile devices it is hard to imagine the iPad to be locked to a specific mobile carrier, but with mobile carriers involve like AT&T it is not totally unthinkable for them to insist Apple in doing so.
BTW: there is already rumors that the US CDMA carrier, Verizon, will have a promotion campaign for their MiFi device targeting iPad users.