Version: (using KDE 3.5.9) Installed from: Mandriva RPMs OS: Linux Instead of a cryptic filename it would be nice to have the subject of the mail as the filename (like it was handled under BeOS). It would allow the user to backup and/or archive at least the entire mail folder and make it easier to use the mails later and/or on another system without an email program. Especially because Kmail can only save a mail with the .mbox extention through the menu. The option to save an email as .txt file would be nice to.
As far as I know KMail uses Maildir for the mail storage (see: http://en.wikipedia.org/wiki/Maildir). The solution you propose would require much logic to ensure that no two messages share a file name or that no messages gets deleted and introduce another format for storing mail. If you need the Subject of the mail run a simple grep in the files ("grep -E '^Subject:' ${dir} -rH" would give you a list of file names / Subjects).
Closing due to lack of feedback and due to the fact that maildirs is not supposed to have the subject in the filenames (http://cr.yp.to/proto/maildir.html).