A new hack has arrived to enable hotspot and tethering usage on the Motorola Atrix 4G without a tethering specific plan. This hack comes hot on the heels of another that enables Webtop usage without a dock.
The device is required to be rooted, and can be completed directly on the device using tools purchased from the android market (Root Explorer and SQLite Editor). The hack can also be completed using free tools on the desktop side, with a bit more work.
The modifications change the access point name (APN) the system uses to connect to cellular data. Rather than using the tethering-specific APN (which is only available to users who subscribe to a plan that includes tethering), the hack changes the device to operate using a non-tethering APN, whiich is available to all data plan subscribers.
The Webtop HDMI hack also works in conjunction with this tethering hack with some further changing of APN settings while the Webtop app is open. Full instructions on how to apply the hack are available at XDA. Be sure to check the whole thread as there is further info on using a desktop and free tools to accomplish the modification without a market purchase.