Maybe W. is surprising everyone again, knowing Clinton will be Clinton and what an event for the contrast to be made clearer?
I think you are absolutely correct. Bush is a high-road guy; he invited President Clinton out of respect for the Office (and Clinton has also been somewhat supportive of President Bush on some issues relating to the war). Clinton mugs for the cameras, Bush keeps the focus on the late great Pope - and the point is made.
It's really brilliant - "reach out" to your enemy and allow him to make himself look like a jagoff, making you look good in the process.
Bush is amazing.