I think you are missing something. It is certainly possible that a new feature in iOS5 might be the ability to stream your media from a home computer, over the internet to your iOS device. But what this is talking about is Apple's huge server farm basically hosting your media collection in the cloud for streaming to your iOS device. Your home computer would not be used in this scenario, so you could definitely leave it at home and turned off or in sleep mode.
As for turning computers off...I commend your desire to save money and energy, however, I think alot of people out there are not as eco-friendly as you and leave their machines on constantly.
I suspect MobileMe will be free. I've been getting it since 2007, and if they were just changing pricing or something rather than making it free then they'd offer some sort of a discount to those who bought it recently, like with iPad2. Instead, you can't buy Mobile Me anymore. I hope I get a bonus for being a paying member at one point, but then again, it being free is a bonus in itself. Plus I'll finally be able to use my @me.com email address with the confidence it'll remain there for a couple of years - didn't want to before because I was always iffy on keeping the service... even though it's pretty good.
I also doubt they'll expect you to leave your computer on for whatever their streaming idea is. It's just a messy solution and it encourages wasting of resources. That's not green. Your ISP could be at fault, then Apple has to deal with service calls for it. Plus its harder to set up. Plus Apple's been in talks with Music Companies about multiple downloads of the same file. Seems pretty clear where they're going, but I do suspect some surprises.