For kicks, I followed the link in Helmet Head’s signature to a Wiki entry on VC and in the Looking Back section found the following: "Most Effective Cycling students confirm that they are surprised to learn how quickly motorists often yield the right-of-way when they try the look back signaling technique for the first time."
In the context of vehicles following behind me, I've always thought that I had the right-of-way, on both left and right turns, and the motorist is merely yielding any desire they might have to run me over i.e. not yielding anything at all.
In the case of a left turn, where I need to cross to the center of the road and if necessary come to a stop, then any following vehicles have a duty to either stop or negotiate a safe passage around me. Anyone that cannot stop is either driving too fast for conditions or tailgating.
Maybe this should be a poll - who is gutsy enough to stop in the center of the road when turning left, especially when there is no center turn lane.