At what point did Chambers became involved in R?

I'm not sure either, but I would guess it's a mix. It seems like it's most of Hadley Wickham's career, for example, and there's clearly some work being done by the RStudio and Revolution Analytics folks, but a lot of the packages are clearly the product of people working on their academic or corporate jobs. My impression is that this is also true for most matlab toolboxes (though obviously not the core language), which doesn't quite explain why people use Matlab (but not Octave), versus R (and not S-PLUS)