- Ensure you have one signature setup in Mail
- Quit Mail
- Create a web page in your favorite editor
- Open the page in Safari
- Save the page as a web archive
- Place the web archive in the ~/Library -> Mail -> Signatures directory, renaming it to match the signature you want to replace.
[robg adds: I haven't tested this one ... just remember to keep your signatures of a reasonable size, and realize that some folks (myself included) may not see your signature due to plain text mail setttings...]

