There has been a lot of questions lately among users looking to purchase an iPad 3G if the device will work internationally. Short answer : Yes, but it might require some work if you are the kind who keeps traveling frequently.
The iPad 3G will come pre-installed with a micro SIM card that is not quite like a regular SIM. However this will work only for the local geography though this is replaceable. This means when you are traveling internationally, you may purchase a new micro SIM from the local carrier for internet use inside the country.
For instance, international travelers to USA may purchase micro SIM from AT&T which are priced at $15 for a 250MB monthly plan and $30 for monthly unlimited download.