I think the timing of the release of the next generation of consoles can have a huge impact on things ...
Even though 5 years is a pretty standard life cycle for a non-market leading console, if Microsoft announced that they were releasing their next XBox console for Christmas 2010 it could do a lot of damage to the desireability of the XBox 360. I think that the combination of consoles selling at a higher price at a similar point in their life and the diminishing returns of more powerful processors has resulted in people expecting their consoles to have a longer life.
One of the problems all console manufacturers will face heading into the next generation is that developers will need a (very) long lead time to produce quality games for launch, while (at the same time) the earlier you announce your console to the public the more likely they will get angry about it.
