To be honest, with 3 power sources and 4 roles, I would have expected 12 classes for the first PHB:
Martial Leader
Martial Defender
Martial Striker
Martial Controller
Divine Leader
Divine Defender
Divine Striker
Divine Controller
Arcane Leader
Arcane Defender
Arcane Striker
Arcane Controller
I'm still hopeful that this is how it will actually be in the first PHB, as it would showcase the great variety and versatility of the system. You might have a four PC team of just one power source or a mix of one or two from each power source. You could compare and contrast how the role is empowered by each of the differing power sources and how each power source is expressed in each role.
Thus, I really really hope the answer is 12 classes.
But, if it has to be 8 classes for the first PHB, well, I can deal with that, although I wonder which eight of the twelve possible combinations they will use in PHB1, if that is the case.
Anyone else hoping the Warlock is not as alignment shackled in 4e as in 3.5e? Didn't someone say that Paladins could be of differing alignments, somewhere? I wouldn't mind something similar for Warlocks. Especially since their background doesn't really mesh well with their alignment restrictions. A PC that gained power through an evil pact I would expect to have a lawful restriction, not a chaotic one, yet lawful is impossible in the current (official) class. Perhaps we can see:
LE warlock (pact with a devil)
CE warlock (tainted by demons?)
CG warlock (fae inspired?)
LG warlock (celestial agreement?)
or something like that? One can houserule a few various changes to make any of these work in the current system, but at present only demonic pacts - as contradictory as that sounds - or similar oddness seems necessary for the background of the class.