[R] apply fun to df returning a matrix

[Soeren.Vogel at eawag.ch]

Hello, a data.frame, df, holds the numerics, x, y, and z. A function,  
fun, should return some arbitrary statistics about the arguments, e.g.  
the sum or anything else. What I want to do is to apply this function  
to every pair of variables in df, and the return should be a matrix as  
found with cov. How can I achieve that? Thanks, Sören

df <- data.frame(x=1:10, y=11:20, z=21:30);
fun <- function(x){
   return(sum(x));
}
# and now???