3.5 Player's Handbook
Bard - these guys have always been fun from a roleplaying perspective, but from a combat mechanics perspective? Not so much. Recasting them as arcane leaders seems like emphasizing the strongest way to play them in 3.x, and can only be a Good Thing.
Barbarian - although the initial 4e defenders (fighter and paladin) are very much heavy armor / stand your ground types, I think there's definitely a bit of design space for a more mobile defender. And if hit points are by role (and it seems like they are), then I think they need to be slotted as defenders. So they're the primal defender (WotC's suggested 'temp barbarian' as a fighter further reinforces this). Class features have got to include rage; probably no armor profs beyond Hide, which means they're going to be STR primary/DEX secondary/WIS tertiary characters (yes, high-CON barbarians are traditional, they're not getting heavy armor, so they'll need DEX or INT for defense, snd INT's not stereotypical for barbarians either -- and they ought to need at least one mental stat).
Druid - WotC has said they're Primal Hybrid in class and role, focused on shapeshifting, and gave some vague hints they'd be able to stand in for a cleric. So while a 3.5 Druid could fill any 4e role, I'm going to guess the 4e is a hybrid Striker/Leader (and can't really do controller stuff or defender stuff well).
Monk - Seems very clear they'll be the Ki striker given a Ki power source and the suggested 'temporary' monk.
Sorcerer - Since these guys basically existed in 3.x to give a spontaneous caster, and someone else that used all of those wizard spells, it's no real surprise that the 4e version is getting a substantial reflavoring -- and at the end of the day mostly exists to give another controller option.