Doing some googling, I see that this bug was reported against plt scheme in 2002: http://www.cs.brown.edu/pipermail/plt-scheme/2002-June/00004...

R6RS takes the approach of allowing (unquote 1 2 3). Section 11.17 of the standard has this example:

  (let ((name 'foo))
    `((unquote name name name)))
          => (foo foo foo)