MS pretty much has to be first no matter what.
Nintendo is awash in money right now with the Wii. Unless the market collapses completely, which I doubt since they could halve the price tomorrow, they have no reason to rush a new console out.
Moreover, as we are seeing with the "360 Wiimote" rumors, the success of the Wii has convinced at least some people in MS that there is a new market that they can exploit. That will be very difficult to do with the current machine - they would need a new machine, dedicated from the start to use a Wiimote like device, in order to tap that market.
OTOH, Sony has a fortune invested in the PS3, and it simply cannot afford to abandon it (the way Sega did the Genesis and Saturn), especially with it's rising fortunes. I find it unlikely that we will see the PS4 before 2012.
In fact, the improvement in the PS3 sales may light a fire under MS's behind to declare victory while it still has a lead, and jump to a new console, I'm guessing in 2010. By that time they could produce a more powerful console for far less than the PS3 debuted at.
The one thing they must be careful of is not to rush to market. 4 years was simply too soon for the 360, and they have suffered for it. 2010 will make it 5 years, but perhaps 2011 will be better (and I suspect that will coincide with the launch of the Wii2).