To average user this means there is a second file (device tree blob) that needs to be loaded by the bootloader and handed over to the kernel. With overlays you shouldn't need to blacklist and modprobe kernel modules by hand depending what add-on boards you have attached to the rpi - now you just uncomment a line in config.txt that is required by your configuration.
With kernel 3.18.3 this transition was completed.
Pogoplug V4 | GoFlex Home | Raspberry Pi 4 4GB | CuBox-i4 Pro | ClearFog | BeagleBone Black | Odroid U2 | Odroid C1 | Odroid XU4 ----------------------------------------------------------------------------------------------------------------------- [armv5] Updated U-Boot | [armv5] NAND Rescue System
Is this change related to the repackaging of http://os.archlinuxarm.org/os/rpi/ArchLinuxARM-2015.01-rpi-rootfs.tar.gz? There was a version from January 2nd and now there is a new one from January 24th.
Rootfs tarball is updated regularly regardless so you have a more current system of you are installing from scratch. If you already had arch installed from before pacman -Suy will bring it up to date - but you already knew that
Pogoplug V4 | GoFlex Home | Raspberry Pi 4 4GB | CuBox-i4 Pro | ClearFog | BeagleBone Black | Odroid U2 | Odroid C1 | Odroid XU4 ----------------------------------------------------------------------------------------------------------------------- [armv5] Updated U-Boot | [armv5] NAND Rescue System