Looking over the grammar of the Flex2 bundle, I think there is room for a little more specificity. The grammar I've attached deals with the mx:Script tag problem brought up by Dave and also adds scopes for little things like
punctuation.definition.tag.xml.mxml, meta.scope.between-tag-pair.xml.mxml, entity.name.tag.namespace.xml.mxml, punctuation.separator.namespace.xml.mxml, entity.name.tag.localname.xml.mxml, punctuation.definition.tag.xml.mxml, etc.
I'm not sure what all those do, but they're probably important to someone.
Inside a Script block, this assigns a scope of source.actionscript3.embedded.mxml and passes parsing on to source.actionscript3. Inside a Style block it assigns a scope of source.css.embedded.mxml and passes parsing to source.css.
Please also note that the bases for this grammar is text.xml.mxml instead of source.mxml. It seems to me that, being XML, MXML is more properly text than source. But again, that's just one guy's opinion. If you all think it should be source, that's easy to change with a search-and-replace ;-)
I think a text file with the grammar is easier to grok than diffing MXML.tmLanguage plist. But if the diff works better, I can provide that also.
Cheers, -Joshua Emmons