There is a way around
Being a Spotify user for almost a year now and I just use the application more and more the Spotify Mobile was something I had been longing for ever since I heard about it. Once installed one of the first things I noticed were that it wasnt able to run in the background. This is a major flaw but I believe we have Apple to blame.
There is one way around this but it requires a jailbroken iPhone. To see how it works take a look at
http://www.youtube.com/watch?v=XdUHeRBF-GY