Ubuntu's keyserver, keyserver.ubuntu.com, has a lot of problems lately. I was trying to add the Pidgin repository to work around bug #389322 but kept getting timeout errors from the server. One of my systems did successfully get the key so all I had to do was transfer it to the others.
These are the commands you can use to do the same. I used a terminal because I couldn't find a way to export the keys graphically with either the Synaptic package manager or Seahorse.
First list the keys:
gpg --list-keys --no-default-keyring --keyring /etc/apt/trusted.gpg
pub 1024D/437D05B5 2004-09-12
uid Ubuntu Archive Automatic Signing Key
sub 2048g/79164387 2004-09-12
pub 1024R/A1F196A8 2009-01-20
uid Launchpad PPA for Pidgin Developers
The "--no-default-keyring" and "--keyring" tells gpg to use only the specified apt trusted keyring. Next, find the key you want to export from the list and specify either the key ID or user ID with the following command:
gpg --no-default-keyring --keyring /etc/apt/trusted.gpg --armor --export A1F196A8>pidgin.gpg
The "--armor" tells gpg to output an encoded text key instead of binary one. The Pidgin package signing key ID is A1F196A8 and it is captured to a "pidgin.gpg" file in the current directory. Then you copy the key file to your other systems and add it to their apt keyrings using either Synaptic (Settings > Repositories > Authentication > Import Key File) or the apt-key command in a terminal:
sudo apt-key add pidgin.gpg
Then you add the repository as shown in the Pidgin download page. To upgrade Pidgin you can use Synaptic (Reload, Mark All Upgrades, Apply), the text-based package manager aptitude (u,U,g), or apt-get on the command line:
sudo apt-get update
sudo apt-get upgrade
Then you just need to restart Pidgin and try connecting to your Yahoo! Messenger account.