ggplot - boxplot and points split by two factors

Hi,
I'm trying to create a boxplot overlayed with points where the data is
split by two factor groups.
So far:

x1<-factor(rep(LETTERS[1:4],5))
x2<-factor(rep(letters[1:2],10))
z<-runif(20,0,10)

data<-data.frame(x1=x1,x2=x2,z=z)

ggplot(data,aes(x=x1,y=z,fill=x2)) +
	geom_boxplot() +
	geom_point()


Obviously I'd also like to separate the points to overlay the relevant boxplots.

Any hints gratefully received.
Thanks,
Tom