Microsoft's not being real forthcoming about what exactly Windows Phone Starter is. But! It seems to be a very mildly stripped version of Windows Mobile 6.5 (officially aka "Windows Phone Classic") for developing countries, much like Windows Starter.
Microsoft wouldn't specifically elaborate on what's missing from 6.5 in Starter 6—just saying it comes in two versions, with Office Mobile 2010 and without—but it's possible, given the language they used that it won't be available for 3G phones. (I know, WTF?) Luckily, it's nothing most people will have to worry about. Just stick with Win7 Phone, and pretend like all this version craziness doesn't even exist. That's one of the bright sides of Windows Phone 7—there's just one. [All About Microsoft via Unwired via Engadget]