works as upper.panel or lower.panel argument of pairs() (package graphics). Displays pvalues better than 0.1.
panel.cor.pval(x, y, digits = 2, cex.cor, p.cut=0.1)
x element of the pairs() matrix
y element of the pairs() matrix
number of non-zero digits to leave at the end
scaling factor for displayed text
p-value cutoff
cannibalized from an example in ?pairs (package graphics)