Version:            (using KDE 4.1.3)
Installed from:    Fedora RPMs

KMail allows automatically adding textual signatures into each sent message. These are configured in Identity settings under Signature tab. The setting 

  Obtain signature text from: Output of Command

allows specifying external command that would output the signature.

To allow more tailored customization and more appropriate content, it would be nice to be able to add additional arguments for this command. Like:

%R - recipient list of this message
%F - folder where the original message was in case of reply/forward
%I - identity role used for message (even this setting is id specific)
%r - recipient list of the original message that is being replied

and so on. After some brainstorming here we could come up with useful
list of options. I guess heavily hooking this to kaddressbook would also be useful, like allowing to use kaddressbook custom fields where personal signature could be retrieved? Perhaps some dbus woodoo or other desktop related stuff like what song is being played or something else? Amount of unread messages in inbox?

Access to kio_http cookie jar could also be useful if the command is using http - for example to retrieve amount of open bugs in bugzilla or whatever.

Everyone could then write their own scripts doing whatever processing for 
guips, thoughts etc that would be customized more for that particular message 
and its recipient(s). Implementing those scripts should not be hard and would not be in scope of KDE anyway.