Trouble is, though, some of the content providors and subscription services might require it.
Napster requires either Win2K or XP.
Not initially they won't. I'm sure there'll be phase-in period of a couple years of some sort. There are some services that will download to Win98 as the minimum requirements.
If the providers do demand that the minimum has to be a Windows Vista, then they're shooting themselves in the foot, because nobody isn't going to spend $300 bucks for a new or upgrade Vista OS unless they flat out buy a new computer that has it already.