The "auto" boot parameter is only supported by the syslinux or isolinux boot menu configuration files on some install media images. I recommend using the longer form "auto=true" which is directly processed by d-i. (The usual definition of "auto" is actually "auto=true priority=critical", but if a preseed file is in use, it's unlikely the priority change actually affects anything, so I don't bother to add the priority=critical bit.)
Jayen:
The "auto" boot parameter is only supported by the syslinux or isolinux boot menu configuration files on some install media images. I recommend using the longer form "auto=true" which is directly processed by d-i. (The usual definition of "auto" is actually "auto=true priority=critical", but if a preseed file is in use, it's unlikely the priority change actually affects anything, so I don't bother to add the priority=critical bit.)