Update:
RStudio (> = 0.97.x) and knitr (> = 1.2) know how to handle file encoding. RStudio passes the encoding of the file as an argument encoding knit()(for example, knit('foo.Rmd', encoding = 'UTF-8')). In short, updating RStudio and knitr will solve the problem.
Please ignore the answer below; this is no longer a suitable solution.
I did it like this (using RStudio):
.Rmd " ", UTF-8knit2html() : options(encoding = 'UTF-8')