Opening this here as a related to: http://drupal.org/node/76379 since the actual implementation of signatures in profile.module is a different issue.
Profile already does everything needed to produce signatures, so I'm not sure where to start to be honest. Quickly I guess the following would need to be done:
1. signature to be set up as a default field in profile.module - might this be the installer rather than the module itself??
2. an upgrade path to move signatures from the user table to a profile field
3. Something somewhere to actually display the signatures in comments...