I think this would be a very useful feature. Especially for drupal social networking sites. For example, a user could select (or it could be turned on by default) -

'Add latest blog post to signature' -> The title of the latest blog post linked to the actual post
'Add latest status to signature' (from the facebook style status module)
'Add xxxxxx to signature' (where xxxxxx is a profile field)

These are common things user's would like to add to their profile and they cannot do it themselves, ie getting the latest blog post or status requires php.