The Macbook Air does not have an Ethernet port. If you're going to do audio or video webcasting, wireless is not the way to go. While it may work for some people, it's a disaster for others. The USB-to-Ethernet adapter at the Apple Store is expensive. Since the reviews on the Kensington USB Mini Dock with Ethernet were so positive, I bought one for my Macbook. It has worked flawlessly.
The Air has only two USB ports. If you use a webcam and an external sound card, there are no ports for a USB-to-Ethernet converter. That's where the Kensington is handy. While it uses one of the USB ports, it provides two additional ports, making a total of four ports available. If you have a USB device that is picky about being connected to a hub, you can connect it to the remaining onboard port.
The dock installed using existing Apple drivers. The Ethernet connection connected promptly. Just to be safe, I turned off the wireless connection on the Air. There is a wall power adapter in case the connected USB devices require more power than the Air port can supply. Once when I used the dock, I forgot to connect the power adapter, and everything worked just fine.
The dock should work well with a Windows PC, but I haven't tried it yet.
The best pricing I see now is from buy.com. They have a good video showing the product at the link below.