I have been stumped by this no start problem for some time. Evey month or so I devote a few hours to searching for new tips and solutions. Today I found mention of the launch daemon and its configuration. Apparently, there is file, /System/Library/LaunchDaemons/org.apache.httpd.plist that contains a key marked as disabled. In my installation of Lion, this file does not exist in that directory.
I was using the server for plain HTML development, but decided to activate PHP. Since I did that, Apache has been broken.