Hot off the Meyers press: How would you implement move, and why? Scott Meyers explains two related issues:

The Drawbacks of Implementing Move Assignment in Terms of Swap

by Scott Meyers

From the article:

More and more, I bump into people who, by default, want to implement move assignment in terms of swap. This disturbs me, because (1) it's often a pessimization in a context where optimization is important, and (2) it has some unpleasant behavioral implications as regards resource management.