From a readability standpoint, I believe fixed width works better since we learn to read in fixed columns. If they are too narrow or too wide, the content is difficult to read.
Fluid width sites make sense if the site is providing Software as a Service. For example, the spreadsheets in Google Docs expands to the maximum size of the browser. This gives you more columns to work with.
Yet, Google (word) Documents are fixed width and doesn't expand or scale to the browser width. Again, this most likely behaves as expected when reading or writing documentation.