shell script question

[Hans <hansfong@zonnet.nl>]

Sorry, I'm full of questions today.

I'm working on a script that can rename filenames. For now I want it to add
an underscore before each capital letter there is in the filename and make
the capital letter lowercase: e.g. FileName --> _file_name.

I tried many things like: 

for a in *; do mv $a `echo $a | tr [A] [_a]`;done

just to try with the letter A. I used quotes in various places, but it
converts A in _ and not _a. I also tried the -c switch with tr, but that
messed up thing quite badly.

So basically my question is 'how to translate one character into two and
vice versa?' Anyone? Thanks for the input.

Hans