This might take things off topic but it still relates to Slacker confusion. I looked further into Pink Floyd and I found that if I looked for albums I get a few albums though I still don't get DSOTM (for example). The albums I can see, seemed to coincide with albums I have in my itunes library - not sure if somehow Slacker picks up on songs I have the rights to. It could also be country dependent. I'm going to talk to Slacker support and get more clarity on this.
