I suspect that Nintendo is weighing quite a few factors inside of the company to determine the timing of their next handheld and console release. Some of this will obviously be dependant on technology being that Nintendo will want to produce a product that excites people on the level that the Wii did; and everything from the processor and memory to the user interface may be dependant on emerging technology becoming inexpensive enough and high enough quality to base a product on.
Other (obvious) factors are things like how they will transition developers and consumers from the current Wii to their next generation system. To a certain extent, Nintendo has the option to enhance their Wii API allowing for some functionality of their next system to be exposed to current developers (if it is running on the new system) in a way that is analogous to the Gameboy -> Gamboy Color transition; but this would have to be something that Nintendo would need to see value in. On top of this, if gamers remain (generally speaking) happy with the level of graphics and user interface available on the Wii it will be difficult to convert them; so the next generation system needs a "hook" that drives adoption.
If you notice, nothing I said suggests a particular time and I suspect that the same (general) thought is going through many executives. In other words, the next generation Nintendo system will be announced when Nintendo thinks that it and the market are ready.
