public key not available for RPM

Wed Mar 14 12:53:55 MDT 2007

When I do a 'yum update', I get a list of the updated packages. It  
then asks if I want to download them. The download begins but then it  
stops because "a public key [is] not available" for one of the  
packages. What do I need to do to resolve this?

(running FC3)
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