This is an interesting question.
The reason why the market leader normally waits to release its console is, obviously, because as far as they are concerned, the gen should never end.
Looking at the past though, launching a console early was always a smart move. The Genesis could take some serious parts of the market back in the days, and I believe that Sony's headstart with the PS1 was one of the main reasons for their success.
The dreamcast had a fine performance too, it was just not enough. And after it was out, Sony's headstart was the best thing that could happen to them. And, last but not least, I think that Microsoft would be dead last this gen if it wasn't for the head start. Launches aligned, the 360 wasn't really doing better than the PS3 for more than a year.
The problem is that when you are late once, you'll probably be late the next time too which was Nintendo's problem after the N64. Imo, you can't start faster than five years after the launch of your last console, or your fanbase will get angry for not supporting their system.
So what Nintendo should do is, in my opinion, being ready. They should develop ideas prototypes and everything for a new console right now and keep a team improving it, so that they are ready to launch their console in less than a year if they want.
I guess Microsoft will be the first to make a move, because they smelled success right now, their console is the oldest and they can afford it, different to Sony. So if Microsoft announces their new console, Nintendo should be ready to release theirs at least half a year after Microsoft. (Unless Microsoft wants to release a new console as early as 2010, which I doubt though.) Like that, Nintendo can probably keep MS down right from the start and give Sony a serious punch because they will probably need a longer lifespan with the PS3 than Nintendo with the Wii.
Of course it is a key point that Nintendo keeps supporting the Wii even after N6 is out in order to keep the more casual gamers satisfied.