The custom appearances for a digital signature are kept in the security directory in the file, appearances.acrodata. The security folder for me (Windows 7 using Acrobat 10) is at [user directory]/AppData/Roaming/Adobe/Acrobat/10.0/Security.
As long the signature has been set up, moving that one file from one computer to another will move the custom appearance of the signature.
For more:
Digital Signature Appearances
Sample Signatures
Creating the signature appearance (Acrobat 9)
Adobe TV: Signing PDF Files and Creating Appearances (Acrobat 9)
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