If you have an intel chipset, and have use any of the ide/pata/sata HOOKs in /etc/mkinitcpio.conf, the 'ata_piix' module will be used for your ide/sata devices.
This may cause your devices to shift names, such as: IDE: devices will go from hdx to sdx. SATA: devices may shift lettering, for example sda to sdc if you have 2 other disks on a PIIX IDE port. This order is determined at bios, and may differ from system to system.
If you have any IDE devices on an intel controller, to check if this change will affect you, and whether or not you have to adjust your fstab and bootloader, check 'mkinitcpio -M' for piix or ata_piix
This is a permanent and unavoidable change within the kernel.
This applies to the 2.6.19 kernels: kernel26 in [testing], kernel26beyond in [extra], and kernel26suspend2 in [extra], and kernel26ck when it is updated to 2.6.19.
Some instructions to move through this change easily are here: http://wiki.archlinux.org/index.php/Fixing_device_names_under_new_ATA_subsystem