public key not available

[Colin Paul Adams]

After upgrading to FC3, I have attempted to perform a yum update.
I have imported the GPGP keys for livna and for fedore.us, but I am
still getting lots of messages of the form:

public key not available for //var/cache/yum/updates-released/packages/xxxx.rpm

What else do I need to do (apart from resorting to turning of gpgchecking)?
-- 
Colin Paul Adams
Preston Lancashire