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.
Digital Signature Appearances
Creating the signature appearance (Acrobat 9)
Adobe TV: Signing PDF Files and Creating Appearances (Acrobat 9)