For what it's worth, some W3C specs use the non-uppercase form. The SVG spec
(http://www.w3.org/TR/2001/REC-SVG-20010904/REC-SVG-20010904.pdf), for
instance, states:
"Within this specification, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be
interpreted as described in RFC 2119 (see [RFC2119]). However, for
readability, these words
do not appear in all uppercase letters in this specification."
-Philippe