Of course screens are getting bigger and wider, so we might all have to change our ideas.
We changed over to fixed width in 2008 - to get more control over our design, IIRC (I wasn't involved in those discussion). More recently - those ads that appear outside the borders of fixed width websites - they are called "skins" - have become ubiquitous, especially on consumer sites.
Most sites are fixed width these days, rightly or wrongly. I am not aware of any sites that have changed from fixed to variable - anyone else know of any?