This question: Customize a User's Signature indicates that it should be possible for CiviCRM to automatically include an email signature if one is set for the sending user. I have set a signature set for the sending user, but nothing appears when I compose a message. I know that I can include it in the message using a template or manually insert a token into my email, but this is harder for my users to remember to do. They expect it to "just work" as shown in the above question. What is going wrong here?
Not a direct answer to your question but maybe intersting for many users: We have developed an extension that provides a set of features for using signatures in CiviCRM.
Please keep in mind that this extension is currently still in beta/dev stage and has an issue when merging contacts.