http://chris.chiasson.name/temp/testa.xhtml
fails to validate even though it is actually valid
according to the W3C validation page that your
invalidation result links to. The MathML was generated
from Mathematica (the makers of which helped design
MathML). Therefore, I am reasonably confident the
MathML is correct.
I believe your validator uses
http://yupotan.sppd.ne.jp/relax-ng/mml2-relaxng20060604.zip
or an older version for its MathML schema. I believe
this schema is incorrect.
In case my server goes down, I have attached a copy of
the xhtml file.
valid xhtml+mathml marked invalid