return vector of element names for vector, matrix or array

Just as as.vector() takes a vector, matrix or array and returns a 
vector in row-major order,
I'd like to write a function to take such an object and return the 
dimension names,
pasted with some separator, as a similar vector.

Here is something ugly cobbled together to demonstrate what I want: a 
function to work
for any number of dimensions.

vecnames <- function(x, sep=':') {
     if (is.vector(x) || length(dim(x))==1) return(names(x))
     else if (length(dim(x)==2)) {
       snames <- dimnames(x)
         return (as.vector(outer(snames[[1]], snames[[2]], paste, sep=sep)))
         }
     else {
       stop("How to handle 3+ dimensions?")
         }
     }

 > x1 <- c(a=1, b=2, c=3)
 > vecnames(x1)
[1] "a" "b" "c"
 >
 > x2 <- matrix(rpois(2 * 3, 7), ncol = 3, nrow = 2)
 > dimnames(x2) <- list(R=letters[1:2], C=LETTERS[1:3])
 > vecnames(x2)
[1] "a:A" "b:A" "a:B" "b:B" "a:C" "b:C"
 >
 > x3 <- array(rpois(2 * 3 * 2, 7), dim=c(2,3,2))
 > dimnames(x3)=list(R=letters[1:2], C=LETTERS[1:3], L=tail(letters,2))
 >
 > # wanted for this case:
 > as.vector(outer(vecnames(x2), dimnames(x3)[[3]], paste, sep=':'))
  [1] "a:A:y" "b:A:y" "a:B:y" "b:B:y" "a:C:y" "b:C:y" "a:A:z" "b:A:z" 
"a:B:z" "b:B:z" "a:C:z" "b:C:z"
 >