Linking NXT and RCX

Does anyone know if there will be a way - preferably an easy way - to 
get the new NXT  Mindstorms to communicate with the old?  An obvious 
design is to combine the old with the new.  For example, considering a 
"Mars Rover" design, use the old RCX to control the motors in the rover, 
and use the new to control a robotic arm.  The need for this is, of 
course, to increase the number of output ports.

I can think of two methods using sensors and motors, but they both take 
up ports.  (Unlike communicating between two RXCs, say.)

Andrew