I don't know of any builtins to do this, but you could probably do this with a little shell script:
for oldname in `ls`; do
# tr translates characters from the first set specified to the corresponding character in the second set
#in this case uppercase to lowercase
newname=`echo $oldname | tr [:upper:] [:lower:]`
mv oldname newname
I haven't tested this, but I think it should work (I advise you test with non-critical data first though).