I think it's wise of Microsoft to tie the Metro UI together with the tablets, Windows 8 and RT. I suspect that once people start buying new PC's with Windows 8 and the Metro UI becomes familiar, we will see a corresponding increase in sales for Windows Phone. The Metro UI will make sense to people and be familiar once they are using it on their desktop PC's and it will give Windows Phone a much needed advantage.
As for the tablets, I suspect what Microsoft will do is sell consumer targeted tablets in the $3-500 range with Windows RT (they really need to change that name though, I think Windows Metro has a nice ring to it...) and professional-grade tablets that are more like laptop replacements, running full Windows 8, that will run from $900-$1500+.
It will be tricky to explain the difference to people, but I think making a clear separation between consumer and business models and the prices should make it pretty clear. I also think a name like "Windows Metro" would make more sense to consumers than Windows RT. Windows RT sounds like NT to me, if I wasn't following the news, I would have no idea what that was.
I think Windows RT will have "light" versions of the most popular apps, like Office, IE, and lots of games, so it will be more than enough for most people. If they do it right, Windows RT tablets will outsell the Windows 8 ones by more than 4-to-1 and they should clobber the useless Android tablets and maybe even become competitive with the iPad eventually.
The key is going to be to do three things that Microsoft isn't traditionally very good at.
#1- Make it clear to people what the product is, what it does and who it's intended for.
#2- Have a clear separation between different product lines so people can easily decide which to choose.
#3- Not try to throw everything except the kitchen sink in to a product. What's NOT included is just as important as what is sometimes.
What gives me hope is that Microsoft has finally shown they are capable of making these distinctions with Windows 7 and with Windows Phone. What I hear about Windows 8, sounds like they are getting even closer. There should only be two versions of Windows 8, a Home/consumer version and the Professional version, both 64-bit.
Altogether there should only be 4 mainstream versions of Windows out there. Windows 8 Home, Windows 8 Pro, Windows RT and Windows Phone. And the apps designed for the Metro UI should be able to run on ALL of them. Obviously there would have to be some changes for the phone version of the apps, but they should make them as minimal as possible. Either make one version of the app, that's "aware" if it's running on a phone/pc/tablet and adjusts accordingly to resolution etc, or make one PC/tablet version and one phone version, but let people pay once and have access to both. If they do that, I think we will see a huge upswing in the number of apps available.