I have a question sort of related to this post. The default value of the CAML variable UserID is the CurrentUserName. Can that be set to different properties or is that fixed? For example, could you make it return the user id from the User Information List?
UserID is simply a parameter which is bound to the CurrentUser CAML variable. SharePoint Designer creates this binding for you whenever you add a DVWP to a page.
UserID actually contains more information than just the name, but SharePoint interprets it differently depending on the context. If you output the value it contains with xsl:value-of, you'll see all of the details.
I'm going to answer this as this is the only outcome, last name, first name. If anyone comes across any other definitive information on this and ways to output it, I'll unmark this as the answer and award them the points.