The only documentation that I know of for esri's REST API is in their online help here:
This is written more from the perspective of a consumer than a provider, but should be hackable.
There are parts of this API that are quite proprietary (some of the output formats) and impossible implement by an open source project unless these format specifications are also made available.
As well, some of the REST APIs aren't especially RESTful. For instance, look at the Feature Service. There seem to be separate "endpoints" for add/update/delete/query instead of using standard HTTP verbs to operate on resources. This puzzles me; I know that esri has some pretty smart people there who understand REST. My guess is that these calls map to some kind of SOAP interface, and esri felt that it would be easier for them and their clients if they maintained consistency between them.
My opinion? If you are only looking at sharing data (not map configuration, metadata, etc) and are not in a rush, you may be better off waiting until Microsoft figures out how they're going to represent spatial data types in EDM. With this in place, you could easily create truly RESTful access to your spatial tables using OData and probably RIA-enabled OData at that. This may be pie-in-the sky for all I know though.