Mobile is already pretty fractured...
It seems desktop only breaks along performance lines, while mobile breaks out like this:
3xx = average battery life, average performance
5xx = average battery life, excellent performance
7xx = excellent battery life, excellent performance
(choose your own modifer instead of "average", and "excellent", but you get the idea).
I think for the desktop what we will begin to see is:
3xx = poor performance, no features
5xx = good performance, hyperthreading, and some more stuff
7xx = excellent performance, and even more stuff than 5xx
(exactly what stuff I don't know, as it appears 5xx will come with PCI-express, cool 'n quiet, 64b?, azalea audio)... I do think they will position the 925 chipset (875 replacement) as a 7xx series component though.
--Alan