Yum public key not installed?

[Bob Goodwin]

Yum update ran for a couple of hours [while I slept] and died as follows:

----------------------   snip   --------------------------------
(279/287): shadow-utils-4 100% |=========================| 931 kB    
00:10    
(280/287): python-2.4.3-8 100% |=========================| 5.9 MB    
01:02    
(281/287): pygtk2-libglad 100% |=========================|  16 kB    
00:00    
(282/287): GConf2-2.14.0- 100% |=========================| 1.5 MB    
00:11    
(283/287): gnome-utils-2. 100% |=========================| 6.5 MB    
00:41    
(284/287): rhythmbox-0.9. 100% |=========================| 2.5 MB    
00:15    
(285/287): metacity-2.14. 100% |=========================| 2.6 MB    
00:15    
(286/287): mlocate-0.14-0 100% |=========================|  46 kB    
00:00    
(287/287): pcmciautils-01 100% |=========================|  36 kB    
00:00    
warning: rpmts_HdrFromFdno: Header V3 DSA signature: NOKEY, key ID 1aa78495


Public key for perl-Net-DNS-0.57-1.fc5.rf.i386.rpm is not installed
----------------------------------------------------------------------
This is the second time this has happened.  I understand what it says 
but I'm not sure
why and I don't know how to fix this?

Bob Goodwin