I have been reading the Terms of Use and Terms of Service for many of the new online music services recently and this excerpt of text from Spotify stood out from the rest:

13. Advertising and use of computational resources

Spotify has a right to allow the Spotify Software Application and the Spotify Service to utilize the processor, bandwidth and storage hardware on your computer or other relevant device for the limited purpose of facilitating the communication and transmission of content and other data or features to you and other users of the Spotify Software Application and the Spotify Service, and to facilitate the operation of the network on which the Spotify Software Application and the Spotify Service runs.

Spotify® Terms and Conditions of Use
Effective as from 14 July 2011

Which basically means that you agree to allow Spotify to use processing power and bandwidth at your disposal. You can not opt of this requirement and at minimum must provide them with 1gb of data storage space (the default setting is 10% of free disk space) on the machine you install the application on.

Of course, you could always turn it off.