nice discussion going on over at mnot's regarding the possible usefulness of a way for clients to indicate the preferred response from a server. typicallly, James Snell already has a proposal registered w/ IETF.
i ran into this issue just recently with my exyus engine. typically browsers will 'expect' to see a 302 redirect to the new resource when sending a POST to the server. however, machine clients (or those using XHR may only want a 204 (no-content) response.
anyway, it's good to see this kind of thing going on. i'm optimistic that this will get some traction. i sure would use it!