As suspected, Microsoft has just confirmed via their FAQ page on Microsoft.com that users of Windows 8 Release Preview will indeed be getting that sweet $40 upgrade deal, as long as they upgraded from a valid Windows XP, Vista, or 7 license.
Confirming this to tech reporter Mary Jo Foley, Microsoft says:
Assuming the customer had a previous version of Windows installed before Release Preview, they’ll be able to upgrade from the Release Preview. They won’t need to reinstall the previous version to do the upgrade; they can just upgrade on top of the Release Preview.
So there you have it – looks like the upgrade to Windows 8 RTM come this fall will be just as quick and painless for users of Windows 8 Release Preview as it will be for the rest of us.