One of my clients has an engineer who quit using his Mac Pro and dual 30" ACDs daily to switch to an MBA for 90% of his workload. He uses a 30" ACD every day for at least a few hours. He really does most of his work away from his desk now, and he swears he is more productive than he ever was before. He says it works "fine."
I connected my MBA to his 30" ACD and played a 1080p video on it for at least 30 minutes without a stutter. However, my MBA was really hot and it didn't seem normal if that makes sense. I really think the 24" LED ACD is the right display to drive with an MBA. My experience is always stunning with the 24" LED ACD. In addition, the 24" LED ACD acts like a docking station and has a better picture than the 30" ACD, IN MY OPINION. I just really feel like the LED backlighting makes the 24" ACD stunning and better. Add with it speakers that sound great, a camera, USB hub, and charger for your MBA!
I would say the MBA would probably be fine doing basic stuff on a 30" ACD, but I don't know how much I would want to use an MBA beyond "basic" if you know what I mean. I would say an MBP with a 9600GT would be the right Mac notebook to drive a 30" ACD for anything intensive at all.
Good luck.