Hadley Wickham recently asked an interesting question on the r-devel mailing list, and being unable to find an existing question on the topic on StackOverflow, I thought it might be useful for it exist here as well.
To paraphrase:
An R function consists of three elements: an argument list, a body and an environment. Can we construct a function programmatically from these three elements?
(A fairly comprehensive answer is reached at the end of the thread in the r-devel link above. I will leave this open for others to recreate the benchmarking of the various solutions themselves and supply it as an answer, but be sure to cite Hadley if you do. If no one steps up in a few hours I'll do it myself.)