According Yahoo Japan, Nintendo is working on a new handheld code name MH.

[Darwinianevolution]

Mar1217 said:
First, it's a rumor.
Second, we already knew there would be a handheld device. Why be so surprised ?

I don't think anyone is surprised, really. The only thing this rumour adds to the rest is that we can say MT instead of NX handheld. But aside from that, nothing.