I do like rolling distros, but there is a definite niggle I have with Arch Linux; bear in mind I update nearly every day, I have seen this issue twice in about a year.
When you go to update (pacman -Syu) you are told a maintainers' PGP key is not valid and prompted to delete your download - all well and good, you are kept safe...
However, the method for fixing this problem is widely reported on the internet, but for brevity I will simplify it for you here, run:
rm /etc/pacman.d/gnupg pacman-key --init pacman-key --populate archlinux
And then your normal upgrades should work again :)
(yes, this is covered in the wiki... but so is a lot of other stuff - if you are still stuck have a look at https://wiki.archlinux.org/index.php/Pacman/Package_signing)