Here's a way to use custom annotation. The justification is straightforward, but you have to determine the coordinates for the text placement by hand. Perhaps you could create some logic to automate this step if you're going to do this a lot.
library(grid)
title <- paste("An analysis of the mtcars dataset ")
subheader <- paste("How does mpg change by\nnumber of cyl?")
p1 = ggplot(mtcars, aes(mpg,cyl))+
geom_smooth(aes(fill="Mean",level=0.095)) +
scale_fill_grey(name='95% Confidence\n Interval',start=.65,end=.65)
p1 = p1 +
annotation_custom(grob=textGrob(title, just="left",
gp=gpar(fontsize=10, fontface="bold")),
xmin=9.8, xmax=9.8, ymin=11.7) +
annotation_custom(grob=textGrob(subheader, just="left",
gp=gpar(fontsize=8, lineheight=1)),
xmin=9.8, xmax=9.8, ymin=10.4) +
theme(plot.margin=unit(c(4,rep(0.5,3)), "lines"))
# Turn off clipping
p1 <- ggplot_gtable(ggplot_build(p1))
p1$layout$clip[p1$layout$name == "panel"] <- "off"
grid.draw(p1)
