If Natal fails to have the predicted impact, MS may well be pressed to release their next gen system earlier than they would like, but supposedly, they're already quite satisfied with the way the current generation has been panning out. Unless things change drastically and the games division starts losing profits, I don't see why such a panic move would make sound business sense.
Ideally, neither MS nor SCE want to release a new console any time soon and would only do so as a reaction to significantly shrinking sales and market share. From a financial standpoint, I'm sure both would like to stay with the PS3 and 360 respectively for as long as possible now that they are starting to recoup some of the losses associated with launching new hardware platforms.
If either jumps the gun first, expect the other to follow shortly. The only problem for consumers is premature hardware releases will only result in sub-par initial releases (like the Saturn for those who remember).
Personally, if MS released a new Xbox in two years, I'd probably wait at least one year after launch before even considering the purchase to see if they repeat design/QC failures of the current gen and until there are at least a handful of games that I would buy regardless of whether it's because they were for a new platform with better specs. Face it; most of the early adopters end up buying more than their share of mediocre or outright duds for the lack of quality initial titles just because they want to buy games for their brand new impressive console.