I don't want to start a bickering battle of whether the MPx200 will ever be upgradable, but when fallowing the link from the main page here, I found that after selecting my country (USA) and my phone, the bottom portion had both "Windows Mobile for SmartPhone 2002" and "Windows Mobile for SmartPhone 2003" available as choices.
There are a couple explanations that come to mind.
1: the team/person behind the design of this section of Microsoft's site just went with a generic setup and didn't worry about what OS would be available to specific phones. Or.
2: Maybe Microsoft has done all they can in preparing the OS for use on the MPx200 and it's left up to Moto/ATTWS?
Just some speculation on my part.
Microsoft SmartPhone Support Site.