> m = matrix(1:6,2,3)
> m
  a  b  c
d 1  2  3
e 4  5  6

> m.l
a d 1
a e 4
b d 2
b e 5
c d 3
c e 6

matrix(m, dimnames=list(t(outer(colnames(m), rownames(m), FUN=paste)), NULL))
 #    [,1]
 #a d    1
 #a e    4
 #b d    2
 #b e    5
 #c d    3
 #c e    6

library(reshape2)
 melt(m)