When a new version of a program comes out, some users will want to install it. When looking online, many blogs and guides suggest adding some repository or installing third party packages from an unverified source. Users don't expect viruses, because there have not been any Ubuntu-targeted viruses.
But installing a third party .deb or adding an unverified repository gives the resource full control over your system, and I think many regular users don't realize that.
The whole security aspect of open source is lost when users are installing just any binary package from the internet, so we should spread awareness about untrusted software sources.
In theory, verifying each deb is signed before installing is possible. In practice, if you look at your /etc/dpkg/dpkg.cfg you'll notice Ubuntu have turned it off as the main repositories don't use it. What little correspondence I can find about signed packages is mostly along the lines of "debsign is pretty broken, don't use it".
As for PPAs, there's already plenty of warning when you add an untrusted repository. apt, Update Manager et. al. will gladly whine every time you upgrade an untrusted package until you follow the non-trivial steps of downloading and adding the guy's public key.
(I say "non-trivial"; while some sites like WineHQ tell you to download a .GPG file and add it via System->Software Sources->Authentication, by contrast the Launchpad PPAs ask to call apt-key from the command line. Worlds of difference from a usability POV.)