I'm sure I'm not alone on this one, but I remember when CoverFlow was basically just a tech demo, and not, you know, the centerpiece of OS X's UI. But of course when Apple picked up the little app to use it for everything else, we lost it -- even if you wanted to use the old simple album-browsing app (and now I'm actually regretting deleting it off of my old PowerBook), you're out of luck.
Unless you use Cover Stream, an iTunes helper app that recreates the old CoverFlow interface outside of iTunes. It's even incorporated the "flip-browsing" that you can do on the iPhone, where you just flip the art over to choose a specific track to play. As for how it actually works with iTunes, we can't say, but at least that CoverFlow interface is still available unofficially.
By the way, I'm planning on doing a roundup (similar to the alarm clock one) of iTunes helper apps like this, so if there are any you want to see included, drop a note here in the comments. I have a few in mind already, but suggestions are welcome.