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 kmail-devel@kde.org or directly to me.

Oh yeah, top-posting
[insert proper flame against top-posting here. see other educated opinions about why KMail should really try to educate its users instead of giving in to bad practice.]
Btw, how's KDE-PIM 4 doing?