The latest one (Nehalem) will no doubt have a longer supported life. Maybe not by much, but as the newer operating systems come out and applications are updated to take advantage of newer features, it will gradually out-perform the older-generation of processors. Just like at first, the G4 with Altivec co-processors didn't show any real performance advantage, but over time it showed why it was more advanced than the earlier processors. I have confidence that the hyper-threading features of the Nehalem will show their advantage over the next year or so.
I just received my 8-core Nehalem yesterday, so I get to do the upgrade from my old dual G5 this weekend...I'm not looking forward to re-installing 5 Logic Studio DVDs plus Protools, plus Waves Platinum, plus other plugins, plus Finale, plus....plus...plus.
But it WILL be fun once I'm done! 8)