An almost identical product from Clearwire can be used with the iPad too.
Here's an excerpt from a story on no contract iPhones:
The mobile hotspot, called the Clear Spot, costs $139.99 to buy, or $4.99 a month to lease, and there is a monthly charge of about $40, said Mike DiGioia, a spokesman for Clearwire.
"It costs a bit more than the 3G service, but the 4G service offers faster speeds — 3 to 6 megabits-per-second downloads, with bursts over 10 megabits per second — and is flexible in that it can be used with many devices," and not just the iPad, he said.
Service plans "don't have usage caps and contracts aren't required, though there are often discounts for people who sign a service agreement," he said.
Link to the story (bad link removed)
Also Sprint has this device from Sierra Wireless:
The "Overdrive" 3G/4G Mobile Hotspot, made by Sierra Wireless and resembling a small router, can connect up to five Wi-Fi devices at the same time, including laptops, gaming devices and even smartphones from other carriers. "Make your iPhone 4G," Sprint brags on its Web site.
The Overdrive costs $100, after a rebate and with a two-year contract. The monthly plan is $59.99 for unlimited 4G usage, Sprint says, and a 5-gigabyte cap for 3G usage.
Taken from this news story on 4G networks/phones. LINK (bad link removed)