[R] adding text to the corner of a lattice plot

[RINNER Heinrich]

Dear R community,

I am using R 2.1.1 on Windows XP, package lattice Version 0.12-5, and
want to add text (sort of a dat-stamp actually) to the lower left corner
of a lattice plot, prefarably _after_ the plot has been created.

Here is a simple example what I do in base graphics:

# base graphics:
> plot(rnorm(100), rnorm(100))
> mtext(as.character(Sys.Date()), side = 1,line = -2, outer = T, adj =
0, font = 1, cex = 0.7)

How can I get the same using lattice?
# lattice:
> require(lattice)
> xyplot(rnorm(100) ~ rnorm(100))
> ???