[R] change axis format for different panels in xyplot in lattice

[levyr@umd.edu]

Dear R users,

My apologies for a simple question for which I suspect there
is a simple answer that I have yet to find.  I'd like to plot
panels in lattice with different graphical parameters for the
axes.  For example, the code 

x<-rnorm(100)
y<-rnorm(100)
z<-c(rep(1,50), rep(2,50))
library(lattice)
xyplot(y~x|z)

plots two panels with the default black axes.  Running the
following code

trellis.par.set(list(axis.line = list(col = "transparent")))
xyplot(y~x|z)

plots the same data without the axes.  Is it possible (in one
plot) to plot the first panel with black axes and the second
panel with tranparent axes?  

Thank you for your time and your attention,
Roy