I would say that depends on how many of the rumors about the system are true so far. If this system is more comparable in power to the 360/PS3 than to the 720/PS4 and Nintendo's goal really is to get 3rd party multiplats on their system, they need to release as soon as possible. Releasing in April/May of 2012 would probably give them a year to 18 months before another next gen system is released, and 3rd parties may start releasing games on those platforms instead.
If Nintendo is expecting to get the same 3rd party games as the 720/PS4 then they can probably wait til November 2012. I think the Wii has enough life left in it to limp along as Nintendo's only home console until Fall 2012 in the West, but I think that Wii will be completely dead in Japan, accept for if/when Dragon Quest X releases. So I'm not sure Nintendo would want to be such a non-factor in the home console market in Japan for such a long time.
