XHTML does not impose any extra requirements on what the value is.

It does, actually. See the DTD:

<!ATTLIST select   
[...]
  multiple    (multiple)     #IMPLIED
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That defines the 'multiple' attribute as having an enumerated set of possible values. Though in this case, there's not much enumeration to the set :)

EDIT: Actually, on re-reading, I guess I may just be agreeing with you...