Many cell phone services offer such a service. I don't know exactly how much it costs, but I know with the correct hardware(even a cell phone with a USB cable or bluetooth) can be used to get your laptop online. Or if you need a new laptop, I know one of the Dell laptops I worked on recently had a sim card slot for cellular access. Net stuff...just don't expect super fast connections.
Oh, as for traveling out of the US, I have no clue how far the service would go. It should be basically fine in the US, but you'd have to see what services(if any) are offered outside the states.