No internet access on MacBook Air, though I can connect to networks
Days later, I've finally figured it out.
The com.apple.mDNSResponder.plist file located in /System/Library/LaunchDaemons/ was corrupted, and for some reason attempts to reset it were not being saved.

I copied this file from a macbook that was able to connect, and, since I'm using Lion, added in the option to allow appending search domains by inserting the line

&lt;string&gt;-AlwaysAppendSearchDomains&lt;/string&gt;


in the following position:

&lt;string&gt;/usr/sbin/mDNSResponder&lt;/string&gt;
&lt;string&gt;-AlwaysAppendSearchDomains&lt;/string&gt;
&lt;string&gt;-launchd&lt;/string&gt;


I then restarted the machine. This appeared to do the trick.