Absolutely not.  XML is XML.  Possibly casual observers might confuse "XML" with "XML 1.0 + XML Namespaces" but a FAQ is all that's needed to sort that out.  The only reason such a confusion would be important is if it made it too impractical for existing parsers to handle MicroXML.  For all the parsers I know of, it would work just fine, or need very minor tweaks.  Do you know of any where that is not the case?

For me (and for quite a few who have spoken up on the list, putting paid to your "everyone" claim), exactly what is needed is something "designed to be a subset of XML 1.0 but not of XML 1.0 + XML Namespaces."