This is a neat idea I came across, it explains (at least it clears up to me) why certain boundaries move considerably faster than others. Think Nazca vs. Pacific. It sounds feasible, but I would like to read the Nature paper for more information.
Click here for the story
If you want to read the actual article here is the citation (and I just found the link [subscription required]):
Saskia Goes, Fabio A. Capitanio and Gabriele Morra. "Evidence of lower mantle slab penetration phases in plate motions." Nature, 21 February 2008.