Yesterday Pradeepto added the TOFU (an acronym for something in German) feature to KMail in the KDE-PIM Features Branch. TOFU provides top-posting -- see the "Insert signatures above quoted text" option in KMail's Composer configuration -- as well as composer window edit options for: Prepend Signature; Append Signature; and Insert Signature at Cursor Position.
As usual, you will find this and other neat new features in $SVN/branches/work/kdepim-3.5.5+
Sorry there are no pre-built packages available for this branch. You will need to compile and install this code yourself.
Feedback and testers needed. Please send your comments to [email protected] or directly to me.