I had spilled something (beer, probably) on the laptop. The keyboard was nonfunctioning and the trackpad was sending faulty data to the processor--the pointer was jumping all over the screen.
I could sort of use the computer if I connected an external keyboard and mouse, except that it wouldn't recognize input from a USB mouse. However, connecting a USB mouse was still better than nothing, as it disabled the input from the trackpad.
The repair went really well. I didn't use the Trackpad Replacement guide, rather the Upper Case Replacement guide. It only took a couple of hours, and it gave me a good insight into how the Air is constructed.
I was a little surprised that neither my replacement trackpad nor my replacement keyboard came with a new trackpad ribbon cable, and yet they both came with a set of trackpad screws. This bothered me a little since the beer I had spilled got into the ribbon cable and started to corrode it.
I did have to open up the case after the repair--the lower case and fan housing became dented and the fan made an awful grinding noise. But a tweak with the spudger (and corresponding bang to the lower case with a hammer) and the dents were smoothed out!
Be fearless. When I started tinkering with electronics I had no idea how they worked, but I did know how to read guides and use a screwdriver. So any fear I had of the unknown or of destroying my computer by accident was mitigated by the fact that I was familiar with components of the process, if not the process itself. From there I only needed a medium helping of common sense and i wouldn't electrocute myself!