What areas of the MacBook Air should I keep clear?
You are correct. While the major fan vent is in the back, it is important that air can circulate under the laptop. This is because the bottom side of the laptop can get hot, and the space between the laptop and the (hard) surface helps release that heat. A soft, squishy surface will just trap it and push it right back up into the computer.And a computer with only one fan vent needs all the help it can get.

That said, you should be OK using the laptop on something soft for most of the time. It's when the fan starts spinning that you really should move to something more solid.
To keep an eye on how hot your computer is,\ and how fast the fans are going, I use the free iStat Pro widget.

Hope this helps.