The protocol between client and server is dead simple, and the server is currently about 400 lines of code. The only reason the server is still in nodejs is because it's quite neat to be able to write transactions on the server-side too and by being in JS, I can just use the same client code as in the browser to do that, with very few changes. Writing just the txn engine in other languages would be very simple too. Next week I'm hoping to write some documentation for the protocol. But it's really just abstract addresses and version numbers and such. No rocket science at all - especially given that Node is single-threaded!