I dont buy the there installed base is too large argument. Sony came in and dominated an already saturated videogame market in 1995 with the PS1. Plus I heard MS is buying rights to music to the sum of billions of dollars so that purchasers can put music for free on there players, or at a heavy discount. And we both know that MS is willing to lose billions to create a large userbase.
I've already converted my music to the iPod format and downloaded a bunch of music from iTunes. Why should I buy a different player and be forced to re-import my music collection and re-buy the music I bought on iTunes. I'm a fan of Microsoft but they seem a day late and a dollar short in my eyes.