Cython is both differently defined and serves a different purpose. First of all, RPython is runnable, Cython is not. RPython has type inference, but more importantly, because it works from live objects, it has Python as a metaprogramming language. It's also much easier to extend than Cython.
Purpose wise cython is for optimizing pieces of code, rpython is for writing interpreters. RPython is quite faster than Cython, but it's also much more restricted, making it a bit unusable for the purpose of "just" speeding up pieces of code (you can't have PyObject equivalents).
I seriously doubt it, considering a) RPython is a strict subset of Python, operating on the in memory representation of Python objects and does aggressive type inference, whereas Cython adds additional features (and syntax) to the language and has it's own parser and no type inference (AFAIK), and b) RPython predates Cython, AFAIK.
Purpose wise cython is for optimizing pieces of code, rpython is for writing interpreters. RPython is quite faster than Cython, but it's also much more restricted, making it a bit unusable for the purpose of "just" speeding up pieces of code (you can't have PyObject equivalents).