I bought my late-2010 11" macbook air a few days ago and I have realised that it doesn't automatically wake from sleep mode when I open the lid. I have also found out that this problem only occurs when I am plugged in (either charging/charged) right after booting up. After that, if I unplug my macbook air and plug it in again, it will wake instantaneously. Please help out by trying this out on your macbook air.