Lattice: Customizing point-sizes with groups

Hello,

I am creating a scatter-plot in lattice, and I would like to customize the
size of each point so that some points are larger and others smaller.
?Here's a toy example:

library(lattice);

temp <- data.frame(
? ? ? ?x = 1:10,
? ? ? ?y = 1:10,
? ? ? ?cex = rep( c(1,3), 5),
? ? ? ?groups = c( rep("A", 5), rep("B", 5) )
? ? ? ?);

xyplot(y ~ x, temp, cex = temp$cex, pch = 19);

This works just fine if I create a straight xy-plot, without groups.
?However when I introduce groupings the cex argument specifies the
point-size for the entire group. ?For example:

xyplot(y ~ x, temp, cex = temp$cex, pch = 19, group = groups);

Is it possible to combine per-spot sizing with groups in some way? ?One
work-around is to manually specify all graphical parameters, but I thought
there might be a better way than this:

temp$col <- rep("blue", 10);
temp$col[temp$groups == "B"] <- "red";
xyplot(y ~ x, temp, cex = temp$cex, pch = 19, col = temp$col);

Any suggestions/advice is much appreciated!
Paul