Single-board CrossFire
The Radeon HD 3870 X2 features a single CrossFire connector at the top of the PCB, meaning you'll eventually be able to add a second card to it to enable 3X or 4X CrossFire modes (depending on whether you add another 3870 X2 or just a single 3870).
Unfortunately driver support for the ATI CrossFireX technology isn't quite there yet, although AMD tells us to expect something in the March timeframe. Given that CeBIT is at the beginning of March we're guessing we'll see it at the show.
As we alluded to earlier, the fact that the 3870 X2 features two GPUs on a single board means that it doesn't rely on chipset support to enable its multi-GPU functionality: it'll work in any motherboard that would support a standard 3870.
Driver support is also seamless; you don't have to enable CrossFire or fiddle with any settings, the card just works. AMD's Catalyst drivers attempt to force an Alternate Frame Render (AFR) mode whenever possible, but be warned that if there are issues with the 3870 X2's multi-GPU rendering modes and a game you may only get single-GPU performance until AMD can fix the problem. In our testing we didn't encounter any such issues but as new games and OS revisions come out, as we saw with the GeForce 7950 GX2, there's always the chance.
AMD insists that by releasing a multi-GPU card it will encourage developers to take CrossFire more seriously. It is also committed to releasing future single-card, multi-GPU solutions but we'll just have to wait and see how true that is.