I'd be very surprised if they ended The Hobbit at film 2 and then treated the third as the extras/extended cut/filler. Likely it's too late to (significantly) change part 1, now we're so close to release, but I'd imagine them 'stretching' out the events of the main plot across 2 & 3 and adding in the filler.|
It's all a little worrying, but not necessarily a disaster. I guess we'll only know when the announcement (or rebuttal) is made, and some explanation given.
As for plot, I've only read the Hobbit once (about a decade ago, at that), and found the plot pacing all over the shop anyway. It wasn't really an 'in the moment' sort of a book, more told at a distance, so I'm not sure any plot line will be ruined by adding more films. If so, as Kalel points out, having 2 films has already detracted on that side.
Why look, it's a blog: