Oooh! Ooooh! I can't even tell you how excited I am to see this post, because I'm planning a similar bookshelf makeover in the near future (plan to post the whole thing on my website eventually).
This is my plan:
Paint the exterior of the bookshelves a very neutral color (was thinking either black, a nice goldeny/tan color, or off-white) and then paint the interior of the shelves a contrasting color (like a nice light turquoisy color...this looks really good together in my head, trust me).
But here is the thing that will really make them look nice: crown moulding.
Cap the tops of the shelves with crown moulding similar to this:
Which gives it the nice built-in look that you see selling for so much money at places like pottery barn.
This is a poor example of what I'm talking about, but it gives you the general idea:
You can also add thin strips of a flat decorative moulding to the front edges of the shelves (for the moulding, I was thinking something like Martha used for this frame, but a much thinner version):
And you can add bun feet or other decorative legs (moulding and decorative feet are usually sold in the same vicinity of your local hardware store...or at least they are in my area).
I hope this helps, and as soon as I'm done with mine, I will post pics!!