ggplot2: Why Does Semi-Transparency + pdflatex Cause Heavier-Than-Normal PDF Fonts?

I've run into a problem where pdf()ing in R and then pdflatex-ing a ggplot2 image causes all of the text on the same page as the image to become emboldened, but only when alpha < 1. Here's a minimal example in R:

require("ggplot2")
"%_%" <- function(a, b) paste(a, b, sep="")
test <- function(filename, alpha)
{
  pdf(filename %_% "-fig.pdf")
  p <- ggplot(mtcars, aes(wt, mpg)) + geom_point(alpha=alpha)
  print(p); dev.off()

  latexDocument <- c(
    "\\documentclass{article}",
    "\\usepackage{Sweave}",
    "%\\pdfpageattr{/Group <</S /Transparency /I true /CS /DeviceRGB>>}",
    "\\begin{document}",
    "Lorem ipsum dolor sit amet, consectetur adipiscing elit.",
    "\\begin{figure}",
    "\\includegraphics{" %_% filename %_% "-fig}",
    "  \\caption{Figure Caption}",
    "\\end{figure}",
    "\\end{document}")

  con <- file(filename %_% ".tex"); writeLines(latexDocument, con); close(con)
  system("pdflatex " %_% filename)
}

test("test1", 1)
test("test2", 0.3)

Comparing the output files test1.pdf and test2.pdf, I notice that the latter document has heavier fonts when viewed in Acrobat or Acrobat Reader. The problem has been discussed here before, but to no resolution.

I can't seem to solve the problem, which messes up the look of reports I generate with Sweave. Does anyone have any insight into it? I'm using R version 2.13.1 on Windows.

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