Adding my voice to the chorus here, with hope that Apple might take notice...
I received my 11" MacBook Air on Thursday of last week. I have an old SMC router that works fine with all my other Apple devices (iPhone, iPad, older Mac mini). With the MBA, I lose the internet connection (but not the network connection) after 5-20 minutes of activity. Last night I couldn't get an internet connection at all, despite being connected to my network.
I took the machine to a Genius Bar appointment yesterday and unfortunately the Genius didn't know anything about this issue. She ran her routine diagnostics on the machine and said that everything was fine. I had run OS X's Wifi Diagnostics and used it to save all the details of my errors, but she didn't know what Wifi Diagnostics was or how to read it. In the end, I said I'd just hold onto the machine for another week and either get a firmware update or return it for a refund. I just can't risk hanging onto a $1200 machine with such a critical flaw, and no comment from Apple about whether they plan to fix it.
I realize, by the way, that I could fiddle around with my router, maybe upgrade its firmware, and get it to work... but obviously that's not a proper solution. I can't go around upgrading the firmware at every coffee shop I visit, etc. I now plan to leave my router the way it is until I get the MBA working alongside my other Apple devices.