What's the intention of opencpu.org compared to other approaches?
In a nutshell: OpenCPU is a layer on top of the regular tools (e.g. RApache, rpy2) that defines a framework and protocol for interacting with R. It handles stuff like object serialization, security, resource control, reproducibility etc, while abstracting away technicalities.
This paper on arxiv goes into more detail on the motivation and design of the system and API.