At my last job, as a Project Manager, I argued (unsuccessfully) for the Developers to be able to use MP3 players while they were working. Unfortunately, the culture in that company was such that many of the so-called business people could not cope with the concept that some people would be able to use their music players, whilst others (e.g. Call Centre staff) could not.
The company I worked at before that (as a Software Development Manager) were open to Developers using their iPods, and I found them more productive because of it - albeit they weren't allowed to plug in their iPods to their PCs for security and copyright reasons.
So to help me in my next job, if I come across this situation again, how would you successfully put the rationale behind allowing MP3 players?