I've got a scoop for you guys: I've just managed to get a leaked build of Spotify.app for Mac which lets you import and play your local music library.
It's not fake guys! Here's the video:
Yeah, you heard that right: you can import your local iTunes / mp3 library into Spotify.
Here are the screenshots:
Still, I'm not sure if this is just a well done mod (I don't think so, it's very difficult to mod an app in this way) or a leaked build, my source hasn't been able to confirm anything.
Anyway, the build is here, on my Mac.
And it's working.