Ive got an 11 inch MBA (bought on launch), and while it does matter for the most part what surface its on, I can easily open it with one hand without the bottom lifting up. This is more of an issue if there a slick surface, where you might have to open it a little slower. And yes, if you open it too fast, the bottom will lift. I have rarely ever found the need to use 2 hands to open my MBA.
The 13inch MBA is much easier to open with one hand. I played with one in the store and the bottom never lifted up. Also, watch the MBA commercial. They open the 13 inch MBA with one finger.. easy shmeasy.