Here is a very interesting article on not only hiding data on filesystems (the article deals mainly with the ext2 filesystem, which should also work with etx3), but also recovering, including from slack space on raw blocks, and even deleted data!

http://www.linuxsecurity.com/feature...forensics.html

It links to the Coroners Toolkit (as has been posted here several times), along with a few other tools I've never seen before. One of them can be found here:
http://recover.sourceforge.net/

Here's possibly even a better solution for ext2 filesystems:
http://e2undel.sourceforge.net/

Here's another link from the Links page - old, and possibly outdated, but a good description of why you may be able to retreive a file, or at least part of it:
http://www.ccl.net/cca/software/UNIX...m/README.shtml

Not too long, and a very interesting read!