the correct way:
1-Connect SEN to a digital pin to control it.
2-Connect RFM12 SS to the SEN of ethernet.
3-Set the Ethernet's SS to 1, and SEN to 0, to talk to the RFM12.
4-Set the Ethernet's SS to 0, and SEN to 1, to talk to the Ethernet.
Yes, RFM12 use the pin 2 (PD2) for interrupts.
but i looked at the http://arduino.cc/en/uploads/Main/arduino-ethernet-shield-schematic.pdf
ethernet shematic and the ethernet don't use that pin :s