I vote for not requiring the outer parentheses unless there is an adjacent comma. That would unnecessarily complicate the simple, elegant proposal.
Otherwise, I would anticipate frequent questions to the help list or tutor list on why something coded like your example doesn't work.
Also, the double paren form just looks funny, like there is something wrong with it but you can't tell what.
OK. I think I can pull it off in the Grammar.
Timing ------
Based on the extensive comp.lang.python discussions when I first floated a PEP on the subject, I conclude that the user community will very much accept the new form and that there is no reason to not include it in Py2.4.
If there is any doubt on that score, I would be happy to update the PEP to match the current proposal for iterator expressions and solicit more community feedback.
Wonderful! Rename PEP 289 to "generator expressions" and change the contents to match this proposal. Thanks for being the fall guy! --Guido van Rossum (home page: http://www.python.org/~guido/)