Thank you for the suggestion. What do you find excellent about this? What would I get in exchange for the added complexity? I must admit that I like the notion of "raw" HTTP, particularly because the server in question will be used primarily by other web services not written by me (it's basically a payment gateway), and everyone and their girlfriend knows HTTP.

I've worked once before with Protocol Buffers, and I can't say I enjoyed it. It ended up being a layer in the middle between the data and my parser, not really serving a purpose, since my parser (written in Haskell) is more strict that the Protocol Buffers specification allows. After that I see little value in Protocol Buffers over Haskell types which are (de)serialized to/from binary data. I don't find Protocol Buffers nearly verbose enough to define a protocol, so it becomes reduced to defining data types/structures, which other tools handle much better, in my opinion.