Arrch:
I've pretty much been working on the same setup with the exception of using RFM12Bs instead of the nrf24s (Form factor issue and availability at the moment). I haven't had any issues with the Ethernet Shield and the RFM12B after adjusting the library to use pin 9 as the SS, but I haven't tried doing that for the NRF24s yet.
You mean the library for the RFM12, or for the ethernet module?
Also, which RFM12 library are you using? I tried the JeeLib library, but the examples don't work.