Arista came out last year with something vaguely(although lacking the modularity of cards)similar to what you've got in mind. I think it was originally targeted for high-frequency trading apps where proximity of the colocation such as 350 Cermak in Chicago makes a huge difference in latency. I never heard much more about it though.

http://www.aristanetworks.com/en/products/7100series/7124fx/