The thing that was stopping me getting an Air was the inability to upgrade to 8GB.
The 2012 models permit that in an online order. For me this now makes the MBA a contender. I would not buy any system with less than 8GB RAM, but that is because of the heavy use I make of a variety of meaty applications all at the same time. The additional memory also gives me better confidence for the future using a computer that can't be upgraded.
Good point, I didn't mention the fact that you can get it with 8GB RAM. It's cool that they did that, I'm sure it will really help folks using Final Cut Pro and apps like that.
For me, though, I'm not sure it's worth almost $400. I'm a web designer, so I generally use Sublime Text 2 or a similar code editor with several files open in it, quite a few tabs open in my web browser, my FTP client, Skype, and occasionaly Photoshop Elements. My current computer only has 2GB RAM, and there haven't really been many times when that hasn't been enough. So I think 4GB will be good for me. I guess 8GB would be good for "future-proofing" it though.