Druids are kinda tricky. In AD&D they were Clerics (in 2e, specialty priests) with generally more offensive spellcasting and the ability to shapeshift. In 3.x, and especially 3.5, that deal got blown out of proportion to a grotesquely overpowered extent. Looks like for 4e, they have to have a Druid be a hybrid to be recognizable at all, but it also looks as if they're only limiting it to a couple of the roles. My guesses would be for the Controller (offensive Primal spells) and Striker (shapeshifting).
That said, I think the Druid will probably be the ONLY hybrid class ever. And I'm fine with the Druid as a hybrid as long as:
a) He's actually an effective representative of both roles, BUT ...
b) ... and I do repeat, BUT, never simultaneously. And, if it can somehow be helped, never in the same encounter. For the Druid, make him an effective Striker when shapeshifting, but disable his equally as effective Controller powers until he reverts back to humanoid form (and for the love of Bahamut, NO NATURAL SPELL! EVER! NOT EVEN IN EPIC TIER!).
The Warlord article prelude mentioned the Shaman as a Leader class, so I'd expect that to be the Primal Leader. The Shaman, and not the Druid as in editions past, gets to handle all the buffing and healing.