For the past six months, pacman's package verification features were turned off by default while we were figuring out the details of our public-key infrastructure.
They have finally been enabled in pacman-4.0.3-2; when you upgrade, you will be prompted to run:
pacman-key --init
pacman-key --populate archlinux
This sets up a local keyring for pacman, and populates it with the data needed to authenticate official packages. This includes five master keys used to authenticate Arch Linux packagers (developers and trusted users), so you do not need to know who joins or leaves the team: you only have to verify those five master keys once and for all. The populate command will prompt you to do so; please do this cautiously by checking the fingerprints displayed against those published on our website.
Then, merge your pacman.conf with pacman.conf.pacnew, that is, enable
package verification through the SigLevel option, and you should be good to go.
For details on the development of pacman and archlinux-keyring, see the blog posts of Allan and Pierre.