John Chambers, one of the creators of R's predecessor S and a current member of the R Core Group, gave a seminar at Stanford last week titled "R, other languages and object-oriented programming". Unfortunately, I was away for the Thanksgiving break and couldn't make it myself, but John has kindly made his slides (PDF) available for download.
The talk covers the benefits of multilingualism: in this case, being able to call other languages from R. It covers in particular the Rcpp interface between R and C++, and the new ability in R 2.12 to create C++-like classes that pass data by reference instead of by value. The slides give a concise introduction to reference classes in R, and note that Rcpp is the first implementation of using R reference classes from another language.
Also, be sure to check out slides 13 and 14 for an important piece of history: the first whiteboard concepts of a new language in 1976, that would lead to the development of S and, ultimately, R.
John Chambers: R, Other Languages and Object-Oriented Programming (PDF)