Yes, the CPU cards from the MDD machines and the Xserves are interchangeable. They're on a daughtercard just like the older G4s, but with a 167 MHz bus which talks to DDR memory. Of course, you wouldn't be able to put the top on an Xserve if you put in a MDD CPU card with the heat sink, but an Xserve processor module would fit just fine in a MDD.

The MDD and the Xserve are the only machines with CPU modules which run the bus at 167 MHz. Sonnet sells an upgrade which will work in either.

Note that the original 1 GHz Xserve and the original 867 MHz MDD had 133 MHz busses; all others had 167 MHz busses.