Using separate services for posts and comments is incredibly common, though, considering how many sites use a third-party service like Disqus or Facebook to provide comments. If you're expecting to load comments in JavaScript (which is really common these days to reduce load time and the impact of link spamming), implementing comments as a separate service where groups of comments are keyed by origin URL or article ID is a no-brainer.
True. I was more imagining discussion sites like this one, where a post is basically just a special parentless comment that's rendered differently, and all normalized tables that connect to one also connect to the other (e.g. both posts and comments are associated with a user profile, both posts and comments have a point score, etc.).
On a plain blog, where "people who post" and "people who comment" are basically disjoint sets, comments can indeed be separated out into their own service, which may indeed be a gateway implemented by a third-party RPC consumer object.
