The CRAN version of ggplot2 now allows users to specify direction=-1 in scale_brewer to reverse the colors. The following produces the same plot as the accepted answer.
ggplot(mtcars,aes(x = mpg, y = disp)) +
geom_point(aes(colour = factor(cyl))) +
scale_colour_brewer(palette="BuPu", direction=-1)