You must have support for the root filesystem (ie ext3, reiserfs) compiled into the kernel. If you do not, you must use init ram disk (initrd).
To find out which filesystems are compiled as what, look for your kernel config file in /boot and search for FS. Like this for my setup:
cat /boot/config-2.6.12|grep FS
To make a initrd look at the link listed above.