Currently pacman scans pacman.conf from top to bottom when it comes to repositories. Let's have this example:

[testing]
Server=ftp://ftp.archlinux.org/testing

[current]
Server=ftp://ftp.archlinux.org/current

For example: Now we have perl 5.8.3-4t1 in testing, and tomorrow we move perl 5.8.3-4 into current and forgetting to remove it from testing.

Everyone who has the testing repository in his pacman.conf on top of current, won't get the newer version from current installed. For testing this doesn't matter much since you keep track of what goes into testing and what goes into current, but for other repositories, like my last gnome 2.5 repository (and the upcoming gnome 2.7 repository too), it will give problems:
I need a newer version of a package because the new package needs it. Instead of waiting for an official one, I create a new package and place it in my gnome repository to fulfill the dependencies. When archlinux upgrades with an official version of the package, people will get stuck on my version if I forget to remove it from my repository.

Could we have pacman to look for the latest version of a package when looking for which one to install?