No-one seems to have mentioned that I, for example, could easily sign any comment [[User:Optim|Optim]] and then it would take a page history and diff to prove that I had tried to assign a comment/vote to Optim. Obviously this would be madness. The solution is a "legal" one rather than technical and is pretty simple:
Rule: - Don't pretend to be another user.
Guideline: - Don't change your signature too often. It makes it difficult for other users to remember who they've built up a feeling of trust with. (example: If Uncle Ed ever becomes Father Ed, I for one would come over all confused :-)
Pete/Pcb21