Are the Actor Model and Process Algebra "equivalent"? I often see casual mention that the Hewit/Agha Actor Model is equivalent in capability to process algebra, like CSP, CCS and ACP. Ie, one can be implemented in the other. Is there a paper that actually contains the proof of this? I'm specifically looking for a citable reference.
This question is too vague to admit a clearcut answer, but it points towards a fascinating research field. The key problem is that neither is the actor model formally well-defined, nor is the notion of equivalency between (formalised) models of computation clear. Defining what expressivity of programming languages means precisely is a wide open problem, particularly for models of concurrent computation. It's unlikely that there is one clearcut notion of expressivity. Instead, language expressivity would appear to be parametric in properties of interest. Joachim Parrow's paper Expressiveness of Process Algebras gives an overview of the problem area, and explains some approaches that have been used to tackle it. There's even an annual conference -- Express -- dedicated to this question.

Process calculi such as CCS, CSP or &pi;-calculi were developed in parts to offer a formalisation of actors, but are themselves of varying expressivity w.r.t. various elaborations of the latter concept. Even seemingly small variations in the operations a process calculus offers leads to huge expressivity gaps w.r.t.  various notions of expressivity.