Just thought I'd post this in case anyone else is experiencing the same problem. For awhile now, I have been running VirtualBox which was installed by adding the virtualbox.org repository to my apt sources. Starting sometime yesterday, the Update Manager started complaining about key authentication problems, namely:
Apparently VirtualBox 3.2 just got released, and it's the first release under the Oracle name (since Oracle's acquisition of Sun). As such, you need to get a new key:
W: A error occurred during the signature verification. The repository is not updated and the previous index files will be used.GPG error: http://download.virtualbox.org karmic Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 54422A4B98AB5139
W: Failed to fetch http://download.virtualbox.org/virtualbox/debian/dists/karmic/Release
W: Some index files failed to download, they have been ignored, or old ones used instead.
Problem solved. I then promptly upgraded to 3.2...
wget -q http://download.virtualbox.org/virtualbox/debian/oracle_vbox.asc -O- | sudo apt-key add -
Hope this helps someone... Cheers,