I live in NE Alabama and in the summer of 2007 (August), we had about 24 days straight of over 100 F with humidity pushing the heat index up to 115 F-120 F. I pretty much tried everything mentioned here then. Didn't lose a bird. It is sudden changes in temp that can wreck havoc. Birds will adjust if given shade and plenty of fresh cool water.
Every August here is a challenge but June, this summer was real bad. July was OK and cooler than normal -- now it is in low 90s but very humid. During the summer months, my chickens pant all during the day and even on the roost at night. I feel bad seeing them panting at night so I run a fan over the roost all night. I have fans I run across the runs during the day as well. I use the "gatorade" or 16 oz coke/ frozen bottles for ice in the waterers (sometimes 2). I provide plenty of shade with parts of my run ALWAYS in shade. I've also tried putting the 2 liter ice blocks in places they gather.
Instead of wetting down the run (as it is too wet & humid here), I just take the hose and let it trickle a stream in a slightly elevated portion of the run so the chickens can stand in a stream of water to cool off-- I only do this when over 100F-- or a high heat index. Dry heat is a lot easier to take than this humid, coking heat.