If you haven’t read it, check out this post on the GMSV blog that calls out the rumour (read this again, a rumour) that BusinessWeek reported that MS is thinking about a portable digital media device "to rival the iPod and perhaps Sony’s PSP as well."
As for time to market, I’d speculate that it would be hard for the Xbox team to get any Xbox game support into a small, multifunction form factor any time soon. But I do like the "vision" that the team at Creative has as they launched the Zen Vision:M player – this is very cool and provides a seamless extension to the PC. Microsoft could provide, IMHO, a more integrated approach to the service and the software as Apple has made through the iTunes + iPod connection. I think that there is a great opportunity for MS to provide greater leadership in the hardware + services space, looking to a great audio-video player like the Vision:M (or new devices from Samsung, Toshiba) in combination with a best-in-class music and video subscription service.
I would also like to see MS step up and, as Apple has done, do more to help evangelize a universal connector for media players: iPod enjoys a sizeable family of peripherals, all possible due to the consistent interface that Apple puts on their iPods. It’s not clear if MS could be that prescriptive with CE companies, especially this late in the game, but perhaps there just needs to be a stake in the ground.