This is my newbie understanding about how the relationship between the first sector of the kernel and the boot process...(thanks riel, zinx, weasel and DrNick for explaination).
in case that sector of code is still written to a boot sector of a disk and executed during boot time. Therefore, it no longer serve the purpose of loading the kernel image from the disk but has handed that responsibility to boot loaders such as LILO or GRUB. The boot loader will find the kernel image and skip the first sector of the image. The reason behind this change that prevent the kernel from being loaded directly by the BIOS from the bootsector is that without a boot loader stage, we would not be able to pass a command line parameter like root filesystem etc to the kernel.