If you're in the market for an Alienware M17x R3 or M18x, NVIDIA has an early Christmas present. The GeForce GTX 580M, one of the most powerful mobile GPUs on the market today, has received a drastic price cut at Alienware that makes it all that more justifiable to buy.
Before, the GTX 580M was a $300+ premium over the AMD Radeon HD 6990M in the M17x R3. The change in price also lines up with the M18x, where two of the GTX 580Ms in SLI is $150 more than a pair of 6990Ms in CrossFire. Performance-wise, they're pretty much equal, unless you wanted to enjoy something like PhysX for example.
But, the downfall of this price drop is that it seems to only effect Alienware-branded machines. Right now, if you're buying another brand, the HD 6990M-optioned laptops seem to be better priced, I guess we'll have to wait and see if NVIDIA drop the price of the 580Ms across the entire range, and not just exclusive to Alienware.