Eleven months ago Intel and Micron, jointly as IMFT, announced intentions to move to 25nm NAND by the end of 2010. For the past few months, IMFT has been shipping 25nm NAND although none of it has ended up in the high performance SSDs we love to cover just yet.
The problem, as with a jump to any new manufacturing node, has to do with yields. In the microprocessor space, new processes generally mean you can't reach your clock targets and you may use more power than you'd like. The more experience you have in working with the process the more you can get these two variables under control and eventually you have a technology you can ship to the market.