Yeah, if you're going to make your own custom pcb with the NRF2401 on it, and want to
use the same pcb for mounting the ethernet shield, there may be some blockage of RF
transmissions, especially if the NRF2401 has a pcb antenna. However, that should only be a
problem for your host node, as I assume the remotes won't mount ethernet shields.
One solution is to mount the NRF2401 way out on the edge, similar to maniacbug's picture,
but that's pretty cumbersome for all your remote boards. I would say mount the NRF2401
inboard, and test the system. If the host node has problems, then use an extension cable
to move the NRF2401 a few inches away from the board.
Alternatively, you could just use a regular Arduino UNO board for the host, with ethernet
shield, and then mount an NRF2401 shield above the ethernet. 3 layers. Then, you can
design the remote pcbs however seems best for your app.