Two Students at TU Vienna (Alexander and Peter) have designed and built a very nice board to interface an FPGA (running JOP) with the LEGO Mindstorms system. Take a look at
The board is a major step towards building FPGA based robots :-)
The design file is placed as open-source and you're free to build your own PCB. For those of you who don't want to solder your own board (like me) we can arrange a production run of several boards with a professional company. Please reply to the list (or to me) when you are interested in such a board.
The very rough estimate on the cost for the board is EUR 100,- However, that depends also on the number of boards we will produce.