Hello All,
This is my first post.  Im wondering the same thing.  This is my second batch of chicks.  My first order of 14 ( 7 NHR and 7 BR) was  July '08.  I have 5 left.  So it was time to get more for the eggs.  They  free range in my back yard.  
My new chicks, 7 Buff Orp came from MPC and are all doing great.  They are 5 wk today.  Its HOT HOT HOT here in TEXAS.  Over 100 every day, from 100 - 110. I have been bringing them inside about noon, and just the past 2 nights taking back outside about 8:30 pm and its still about 97.  I have one huge frozen bottle in their 4x4 pen. Its in the shade at all times.   I remember with my first batch, one day I went out and they were panting and spreading wings, scared me! so I kept them in the house in the afternoon, but I dont remember for how long.  I put them out in my chicken house at about 3 mo.  
I also wet the yard around bushes and such for the old hens. ANd wet the ground in the chick pen, and have a little flower pot saucer that I keep water in , in case they want to wade in it. 
Im wondering at what age can they stand the heat to stay outside all day.  
I love this forum and am already thinking about getting more in the fall when its not so hot.  Just havent decided what kind to get yet, I may get a straight run and have some roos to eat.  
thanks!
pam in TX