For all you nay sayers.. I'm running Lion on the original MBA Rev A. (upgraded the drive to a 128GB SSD) Light duty work, browsing, office, media comsumption, plus Lightroom 3, Photoshop Elements and some HDR software.
It runs quite well.. quicker than SL.. and as fast as I need it to.
I've been itching to get a new 2011 MBA but for now, I really can't see any benefit in upgrading other than if I want to run a virtual Windows machine well. I have a MacMini for that.