Back when the Motorola Atrix was released, the $399 Lapdock accessory was also introduced to make use of the webtop features the Atrix brought along with it. But it came in at $399 and while it eventually came down from there, many folks simply weren't interested in it.
But, if you always wanted one but never didn't want to fork over a lot of cash for it -- AT&T will sell you one now on the cheap. $50 cheap in fact, making that a savings of $350 when all is said and done. Plus, with a few mods here and there -- you can get it to work with other devices.
Update: As pointed out in the comments below, AT&T has changed the online price back to $100. Still a much better price than the original $399, but double the 50 dollar fire-sale style price it was listed at earlier today. You certainly have be quick on these type of deals!