Serious distributions try to protect their repositories cryptographically against tampering and transmission errors. Arch Linux, Debian, Fedora, openSUSE, and Ubuntu all take different, complex, but conceptually similar approaches.
Many distributions develop, test, build, and distribute their software via a heterogeneous zoo of servers, mirrors, and workstations that make central management and protection of the end product almost impossible. In terms of personnel, distributions also depend on the collaboration of a severely limited number of international helpers. This technical and human diversity creates a massive door for external and internal attackers who seek to infect popular distribution packages with malware. During updates, then, hundreds of thousands of Linux machines download and install poisoned software with root privileges. The damage could hardly be greater.
The danger is less abstract than some might think. Repeatedly in the past, projects have had to take down one or more servers after hacker attacks. The motivation of (at least) all the major distributions to protect themselves from planted packages is correspondingly large and boils down to two actions: one simple and one cryptographic.
Read more at Linux Pro Magazine