You can register up to 10 phones (3G models with AT&T accounts) to work with the device, but it only supports four simultaneous calls or data sessions. It has a roughly 465 square metre range, and calls originated from it are automatically handed over to AT&T towers once you leave the house/office.
The MicroCell page also confirms an unlimited calling option where domestic US calls made using it won't deduct from your regular minutes. Problem is, AT&T hasn't coughed up what the pricing will be. An apparent leak over the weekend suggested it could be $US20 per month ($US10 if you have an AT&T Internet or landline service; free if you have both). No word yet on how much the device itself might cost.
Travellers will be sorry to hear that, as with Sprint and Verizon did with their femtocell units, AT&T's device has a GPS chip, so it can't be used out of the country. For that reason it's recommended to be placed near a window.
You'll need to have a net connection of at least 1.5Mbps down/256kbps up, and find your ZIP code in the availability checker at the site. [AT&T 3G MicroCell Site | 3G MicroCell Manual (PDF) | via @bugdave]