I also want to add that IMHO the 3.5 Warlock was not introduced because of the concept, which could have been easily covered by Wizard, Sorcerer or even Cleric with an appropriate choice of abilities, but because of the mechanics. It was a clear attempt at getting away from vancian magic and test a spellcaster that would have no daily limits on his magical resources.
I hated it for many reasons in 3.5.
But now that 4e is a different ruleset, and that EVERYONE has daily abilities together with at-will abilities, there is no major mechanical difference between Warlocks and the other characters. So the differences are all in the character concept, and of course in the specific abilities that are being designed based on the concept.
And I have to say I like this 4e concept much more than the 3.5 one. It's much more well-round thought out. It's not too narrow, because there is mention of actually different things, from pacts to souls and perhaps even truenames (and why not blood magic? at this point, the Warlock could be a receptacle of all the "twisted" methods of gathering magic powers). This makes the concept wide enough to fully deserve its own core class.
As I said before, my only regret is that the introduction of the Warlock and the Warlord is being done at the expense of the loss of the Druid, which I think it's really sad, since the Druid is a long-time established core class.