The RTF 2007 specification is now available. Word 2007 added as many control words as Word 2000, XP and 2003 combined! The .doc spec is 4.4 MB, while the .docx is only 881KB. Admittedly a zipped version of the .doc spec is somewhat smaller than the .docx version. The math section starts on p. 124. It includes all of the material in my math RTF post as well as many examples, the BNF syntax, and much additional documentation.
For the Office Open XML formats, go to Ecma-376, where you can download ECMA-376.pdf. This is a large file containing several parts that explain Microsoft Office's Open XML formats. The main reference part starts on p. 778 of ECMA-376.pdf and is titled Office Open XML, Ecma TC45, Final Draft, Part 4: Markup Language Reference. Chapter 2 is WordProcessingML Reference Material and starts on p. 5 of this part.
Chapter 7 is the Shared MLs Reference Material and Section 7.1 documents Office math XML (OMML), starting on p. 5744 of ECMA-376.pdf.
For easy reference, the following table summarizes the sections of this document that define the OMML math elements. The section and page numbers refer to Part 4, not to ECMA-376.pdf. The math objects described in my math RTF post have "Function" in the corresponding Purpose entry, and the arguments (like numerator and denominator) have "argument" in their Purpose entry.