Wasn't there already a rumor a few weeks back that Netflix might be coming to the 360 and PS3? ...or at least that they were testing the waters with that?
It'd certainly be to Netflix's benefit to attach themselves to any prominent service or device they can. Their streaming is only available through IE right now (as it uses ActiveX), but they're moving to stretch to Firefox and OSX/Safari as well. It would behoove them to be attached to the Wii and PS3 as well, (Heck, even the PSP and DS and iPhone/Touch if they want to try for it.) and it would certainly be no real issue to whip up a streaming client on any/all of them. It would reach a lot more people, reach them on the road, and grab a whole lot more subscribers. (The portables might even make the most sense for them, as they'd appeal to a different market than the console crowd, and involve less bandwidth to stream to them.) They might have to shift the subscription level at which you can get unlimited streaming, perhaps...
Even if they do go forward with a Microsoft partnership first and advance a plan on the 360, is there any real indication that they'd be doing this exclusively? They might be able net a nice marketing splash with it right now, but it certainly seems like a way to only limit growth. Netflix has a popular and even well-fitting name to promote, and they'd be well-positioned to outgrow any of the other streaming startups; they should be getting to as many markets as they can as soon as possible, so long as they can do it well.