I have this SAME problem.... I just bought my Acadia like 2 weeks ago, a 2008 SLT. It didn't do it originally, so something has definitely changed and caused it to happen. I even drove to the dealership and had them give me the keys to another new one. The other one fired right up without ANY delay whatsoever.
Today, mine is in the shop and they have re-flashed the computer to see if that would fix it... NOPER! So now they are hunting around looking to see what else they can find. If they figure it out, I'll post again.