I think the reason they have expanded the definition of "core" goes beyond what is needed to play or even what they want people to purchase. They are making it so that anything, in any of these books, can be used by 3rd parties through the GSL.
I hate to crush any hopefuls here, but the Bard is just not going to be in the PHB. They have confirmed 8 classes, and screenshots of the of DDI show 8 classes and the Bard isn't one of them (Fighter, Paladin, Ranger, Rogue, Warlock, Cleric, Warlord, Wizard). However, it does sound like the Bard is complete, so we will probably see it online a month or two after the release of the PHB (and then probably published later in an Arcane themed book). I'll be interested to see the Bard, but something tells me it won't be the "jack-of-all-trades, master of none" that it was in previous editions. Of course, that dubious honor did make the Bard an underpowered class, so I can't say that I'll miss it. He'll probably still have a good, eclectic skill selection.