I am used to mail acting like the standard mail reader and a bare-bones text only user agent, and mailx acting like a more fully featured system, with the single killer feature that you can attach arbitrary files to it with with 'mailx -a foo'.

On Debian systems, mail and mailx are just symlinks to the same binary, and by default you get the basic version with mailutils. With the mailutils version the -a flag appends headers to the message, which I rarely have any need to do.

If you want the appropriate mailx behavior on Lenny, install the heirloom-mailx package. On Etch, install nail.