A bit off topic, but has anybody though about simply using Arduino http://arduino.cc/en/ together with an Ethernet shield http://de.rs-online.com/web/cpd/6961661/ and then just making one new shied for every FPGA that shall be plugged on top?

This could be the 100€ "backplane" with Ethernet and each FPGA board would need nothing than the FPGA and power. This could also keep the cost of the daughterboard low.