getting your Library List on your phone/pda?

Have you ever bought a CD and discovered, once you got home, that you you already owned it? Embarrassingly, I have. Not infrequently, either. Now, I believe in supporting my local music store as much as the next guy, but that's a little bit ridiculous.

What I'd love is to have my music library metadata (not all of it, necessarily, but at least artist/album) on my iPhone, so I can see what I own, and what's missing, while prowling the racks at my neighborhood music store.

As long as I'm dreaming, it would be an easy sync operation, so it would stay current as my collection grows.

So as they say, is 'there an app for that?' Or perhaps some other technique that isn't too overly tweaky and manual?

Thank you for your help and experiences.