No JOINs in UPDATE ... FROM?

Folks,
In 7.1 RC2, I've been trying to use the following JOIN syntax:

UPDATE assignments SET status = -1
FROM assignments JOIN orders ON assignments.order_usq = orders.usq
WHERE orders.status = -1;
However, I get an error of "Relation assignments referenced twice in
query."  
Now, I can (and have) re-phrase the query so that PostgreSQL will
accept it.  However, I was under the impression that the above was
standard SQL92.  Am I mistaken?  Or is this form something that just
hasn't been implemented yet?
                -Josh

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