Hi Allan,
Are you sure it was a "-Syu" and not an "-Sy" followed by installing something (which many people now have discovered is bad...). The readline and bash packages were one after the other so readline was in the [core] repo without base for about one second. It is unlikely any mirror did a sync of the repo db in that time.
Ok, I admit it was something like that ;) It was my older laptop, I have / mounted on a 4 G partition, and I had 1.3 G free. I upgraded after a couple of months, so I had to download 700 M of packages with ~2 G installed dimension. So I decided to do it piecewise, starting with the kernel and other big pkgs (texlive, openjdk...). Pacman downloaded and installed kernel and dependencies, with readline 6, not touching bash 3. Then -Scc, and I went on with another group of packages. After I while I got an error message (something like readline.so.5 not found on filesystem) and I was not able anymore to login as root, nor to halt or reboot. Then I looked in the bbs and found that the problem was that I did'n install bash4. Booted from a live cd and fixed it. Still, pacman let me install readline 6 without upgrading to bash 4. I consider it a bug (even if the policy and common sense say that you should upgrade with -Syu). Cheers, Marco