Apparently Alyssa Milano used her Twitter account to verify her Google+ account.
XFN 1.1 defined a number of relationship attributes for links, one of which is rel="me"
rel="me" A link to yourself at a different URL. Exclusive of all other XFN values. Required symmetric. There is an implicit "me" relation from the contents of a directory to the directory itself.
Clearly, we need rel="me twitter" for situations like this.