I put an ugly dent in the narrow front edge of the top (keyboard) case of my nearly new (mid-2011) MacBook Air.
iFixit Install Guide is excellent, but a couple steps should be modified or added.
"Step 3 - Battery" Your guide suggests unplugging the battery, when in reality it NEEDS not only to be unplugged but also REMOVED.
"Step 37 - Track Pad" needs to be added. The track pad needs to be transferred from old top case to the new one, and no guidance is given. It's a bit tricky, with the connector located under the stuck-together ribbon cable, and a track pad set screw which needs to be properly adjusted. Also a warning about those extremely delicate track pad hinge screws.
"Mystery Clip" When I removed my logic board a tiny 1/8" long spring clip appeared from nowhere, and I have yet to figure out WHERE it belongs. The good news is that I'm typing this from my Air right now and it's working great, thank you!