Is the optimisation fence behaviour of a CTE (WITH query) specified in the SQL:2008 standard? If so, where?
I think it's an implementation detail. 


  A conforming implementation is not required to perform the exact
  sequence of actions defined in the General Rules, provided its effect
  on SQL-data and schemas, on host parameters and host variable, and on
  SQL parameters and SQL variables is identical to the effect of that
  sequence. The term effectively is used to emphasize actions whose
  effect might be achieved in other ways by an implementation.1


I think an implementer could evaluate a common table expression 20 times, even in 20 different ways, and still have a conforming implementation. The only relevant issue is whether "its effect . . . is identical to the effect" of the sequence of actions defined in the General rules.

[1]. Section 6.3.3.3, "Rule evaluation order", in a draft of the SQL 2008 standard, having the local filename 5CD2-01-Framework-2006-01.pdf, p. 41 I have no idea where I got it.  Google might know.