In reply to Roby:
Bishop Bouldering Survival Kit available here:
http://www.rockfax.com/shop/usa_shop/index.html
It will still be hot in September - 80's F during the day.
The Buttermilk Boulders (home of the Mandala) should be cool after 5.30pm when the katabatic wind brings cool air down from the mountains.
The Mammoth Area (8,000ft plus) will be good during the day.
Drop in my office when you arrive in Bishop at 174 West Line Street and I'll give you much beta.
best regards,
Mick