This sort of API is not RESTful at all, regardless of whether the method name is in the HTTP header or a form variable. It's still an application-specific protocol that requires out-of-band knowledge to use. REST resources are supposed to be self-describing, which doesn't make much sense for an API like this. RPC is what is required here, so just tunnel RPC over HTTP in whatever way is most practical. And don't call it REST.

If you want a bucket of cold water over the head on this topic: http://roy.gbiv.com/untangled/2008/rest-apis-must-be-hyperte...