Hello. I ordered a 13" 1.86 w/ 4gb of RAM from macmall on Nov. 17 around 3pm eastern time. I received my air Nov 18th, around 2pm. I chose fedex overnight, simply because for whatever reason it was free.
Out of curiosity, I went to macmall just now and configured a 13" mba just like mine to see if the prices had changed. They hadn't. However, to my surprise if you continue to the point of entering your zip to figure shipping, it looks like overnight is FREE again! Order now, and enjoy.
Edit: On the far right side of macmall.com at the top under the "electronics" menu tab, there is a link to a map that details out the last day to order items in order to get them by dec 24th.