Michael[tm] Smith, the guy who looks after the W3C HTML validator says here:
It's not an error, it's a warning. And it's emitted because the
content is being served as HTML instead of with an XML MIME type, and
HTML parsers don't know anything about namespaces -- to put it in XML
terms, every element name is a local name -- and so in HTML, the
literal name of that element is "g:plusone". And that name can't be
represented in XML because XML doesn't allow colons in the local
names. So the spirit of the warning is to say, In case you ever want
to serve this content as XML instead of HTML, you have an element name
in it that's not allowed in XML.
He's talking about the element g:plusone
but it's the same issue.
But I disagree slightly. Colons are valid in element local names in XML 1.0. They're only disallowed in XML 1.0 + namespaces So the warning message could definitely be improved.
UPDATE:
I previously offered a workaround based on document.write, but as Jan M points out in the comments, IE has it's own ideas about what to do with elements with a colon in their tag names, so it didn't work there. Instead, I recommend following Jan's answer.