Since my Fedora 13 upgrade, I kept getting errors when installing Picasa:

Transaction Check Error:
   package nss-softokn-3.12.4-19.fc13.x86_64 (which is newer than
   nss-softokn-3.12.4-17.fc13.i686) is already installed

The solution, which I found here, is to first do

sudo yum downgrade nss-softokn