Making an Accusation Stick
I think David Kurtz is right that the Clinton campaign is trying to push an "inauthentic" narrative on Obama. It's a tactic that could work but for the ham-handed way they are going about it.
First off, if you are going to follow the Rovian strategy of attacking an opponent on their strength you can't do it using a full-frontal assault. You first have to lay the foundation for the narrative by suggesting over a significant stretch of time that there are reason's to question Obama's authenticity. Clinton has not made this argument before (at least not to my recollection). So for them to leap on it now with the plagiarism charge again strikes of desperation.
(Unfortunately, it's not to late for the GOP to start making this charge stick, which is why the Clinton campaigns accusation may help McCain more than it will help her.)
Now, I think the Clinton campaign was starting to make a more effective argument that a President needs to do more then just give good speeches. Obama has substance to back up his campaign, but few would deny that inspirational rhetoric has been his main weapon in this campaign. Undermining that rhetoric with an updated version of "Where's the Beef?" could work.
But this wild charge of plagiarism could destroy any effectiveness that tactic would have had.