Micron showed off a fully functional DDR4 module back in May. Others are working on the next-gen system memory, including Samsung, which showed a 16GB DDR4 module at the Intel Developer Forum this week. The registered DIMM is laden with 30-nm memory chips rated for operation at an effective 2133MHz.
X-bit labs saw the module in addition to a roadmap that outlines Samsung's DDR4 plans. The new memory type is expected to hit 2400MHz next year, 2666MHz by 2014, and 3200MHz some time after that. I guess it's time to start talking about memory speeds in terms of GHz rather than MHz. Looks like the required voltage is set to remain steady at 1.2V, although the DDR4 spec hasn't been finalized yet. According to X-bit labs, the standard is 98% complete.
While it's unclear what's holding up the standard, platforms supporting DDR4 aren't exactly imminent. Intel will reportedly hold off on using the new memory type until 2014.
Unlike DDR3, which supports multiple modules per memory channel, DDR4 appears to be limited to one DIMM per channel. That may be disappointing news for people who tend to upgrade their systems gradually over time. Seems like folks don't add system memory as often as they used to, though.