http://drupal.org/node/130366 fixed a major complaint with the signatures system: it made signatures dynamic, so that they weren't coupled to the comment body and so that users could update their signature once and have it affect the entire site.
Now comes the second part: addressing two remaining issues with the signatures system:
- Signatures are rendered on forum replies, but not forum topics.
- There is no way to just plain turn signatures off.
My solution to both of these problems is going to be adding the ability to turn signatures on and off per node type. If they're turned on for a node type (e.g. forums), then both the initial node and its replies will have user signatures.