The latest (somehow weird) rumor I heard: Boxster S gets the Cayman S engine starting with MY 2007 (summer 2006).
This doesn't seem to make sense but it came from a pretty reliable source.

The Cayman (non-S) seems to get the Boxster engine with 250 HP instead of 240 HP.

And, like it or not: the Cayman doesn't seem to sell as good as Porsche has hoped for. Of course I can't talk about all markets and all dealers worldwide.
So the question is: why would Porsche put the Cayman S engine into the Boxster S since this engine would be one of arguments to actually buy a Cayman S? Strange.