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The prose description of xs:double and xs:float says that the lexical representations "INF" and "-INF" are accepted; the regular expression also allows "+INF".
Thank you, good catch. In preparing the current status-quo documents I have gone ahead and unilaterally changed the relevant prose; it should have changed when we updated the regex and EBNF.
The unapproved fix for this bug is in the LCWD January 30, 2009.