I am confused with this too

, I installed on my wifes laptop last night (Vista Home) with the run from windows method. Since I installed the W7 Pro I had to do custom install. No problem used the handy windows easy transfer and it took care of her docs, email accounts, settings etc. I simply had to reinstall MS Office and it was all there. *At first he emails were not there, but since she is on my compaines exchange server it synced the 200mb in about 5 mins!
Anyway, today I installed W7 Pro on my desktop which was running W7 RC, I was concerned it wasnt going to let me(from what I have read), but I booted from the CD, formated the raid array, and installed with out question. I put the CD Key in when asked too. Now I do have a Windows XP drive so IDK if because it saw it I was allowed to proceed, it did not mention it, other than showing the drive as an option to install on.
Later today I will be installing my 3rd pre-ordered copy on my laptop, on a new drive (which has W7 RC on it currently) so hopefully it wont give me an issue. I have a different drive with XP on it that I am leaving as XP.
I also heard the upgrade could deactivate your OS you are upgrading from, but I would think it would have to warn you! I have a Vista OS I was going to upgrade from if it asked, but it didnt so no worries!
So to me it seems the retail version will allow you to uninstall it from your 1st computer and say install it to you new build next year and the upgrade version will only allow you to install it to one computer. Of course only installed on one computer at a time? I have installed xp upgrade and oem versions on a new build and when I activated I had to call so I told them my motherboard died and I had to replace it, they activated it for me that way 2 or 3 times. Of course it was true, but the last time I replaced my whole computer and still they activated it.