There is one on the market - the Creative Blaster GeForce 2 MX. But before you get excited, this card has worse performance than the normal GeForce 2MX. This is because MX DDR cards have 64-bit memory busses rather than 128-bit memory busses present on the SDR version. The total 'theoretical' throughput is the same, but because DDR is not quite double the speed of SDR, it is slightly lower, hence worse performance.
I've been told it would be possible for a company to make MX's with 128-bit/DDR memory, but the price of the chips would bring them into direct competition with the GTS, so I don't think anyone has plans to do so at the moment.
That said, I have heard plans for a 64MB MX pretty soon. Plus, there's bound to be a GeForce 3 MX out sometime next year, and this will probably be cheaper and faster than the GeForce 2 GTS.
~ I'm not AMD biased, I just think their chips are better. ~