When running an apt-get update on my phone, I got the following error:
Reading package lists... Done W: GPG error: https://downloads.maemo.nokia.com ./ Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY ADB4438160A655EF W: You may want to run apt-get update to correct these problems
Running update again does of course not help and will give you the same error again.
Trying to import the key with
gpg --keyserver subkeys.pgp.net --recv-keys ADB4438160A655EF
does also not work, because the key is just not there.
So after trying this, I went the hard way and disabled all package sources in /etc/apt/sources.list.d/hildon-application-manager.list, running an apt-get update after enabling each separately. The offending line was
deb https://downloads.maemo.nokia.com/fremantle1.2/ovi/ ./
Which I simply removed, since I'm on fremantle 1.3 anyway.