I work on multiple computers throughout my workday as my clerical team switches desks. I use Outlook signatures for templates for common emails I send; I have about ten "signatures" (form letters really). It would be great if Outlook would store my signatures in Exchange, but it doesn't. I had just received a new CPU at one of my workstations, and I was looking up how to move my signatures to the new computer, when I found a great website on Microsoft's very own site. Don't scoff until you read it. The secret is holding down the Ctrl button while you click Options>Mail Format>Signatures. Who'd've thought?
The author is great and I think we'd be friends if we met, so please go right to Ellen's description at: http://blogs.office.com/b/microsoft-outlook/archive/2008/07/11/how-do-i-back-up-my-signatures.aspx