Yosemite and Bishop

Hmmm, september would be pretty hot in bishop but not unreasonable in the high granite areas of the buttermilks, rock creek, and chasing shade in the tablelands.  When I was there for the month of november, there were cars lined up at the happy's for night bouldering sessions almost every day to avoid the heat.  Yosemite I haven't climbed in much, but I imagine september is a pretty prime time to be there and with a new guidebook available for the bouldering it is certainly more accessable for visitors.  As with most american climbing destinations, a car is almost a necessity.  Just to start with, for example, Yosemite is at least 5 hours from the nearest airport. 

bring a good tent. and buy the bishop bouldering guide beforehand if you can.

Since no locals have offered any help... To be honest, September is going to be pretty hot in both of those places- summer is scorching, and the heat can linger well into early fall. If you could push it back to maybe October/November for Yosemite, or November/December for Bishop you would probably be much happier with the conditions, particularly for bouldering. However, if you're stuck with September and you're really psyched on one of these two spots you could make it work - there's plenty of shade in Yosemite Valley, and Tuolumne Meadows (part of Yosemite National Park) will still be in season and is definitely worth checking out, although it's not really a bouldering destination. If you head to Bishop you could hit the higher elevation areas - Rock Creek and the Sherwin Plateau, for example, but the places you've heard about (Happy Boulders, Buttermilks) will still be very warm. Night sessions are an option, as is sticking to the shade, but really your best bet is to try to come a bit later in the year for these two destinations. As moonshine pointed out, a car is indeed almost a necessity- if you fly into San Francisco you'll be 3.5 hours from the Valley and around 6 from Bishop, and spending a month living at one of these areas will be much easier with a car, especially in Bishop- once you got there I think you would be ok without a car in Yosemite but getting around the Bishop area would be a pain.