Hey.
It looks OK (for booting Nitdroid without EXTCARD installed), but I assume You do have an SD card in Your device?
Then try changing this line
ITEM_DEVICE=mmcblk0p2
to
ITEM_DEVICE=${INT_CARD}p2


And I do see other difference, but not sure about their responsibility. Please see my files (all working):

~ $ cat /etc/multiboot.d/10-NITDroid-mmcblk0p2-2.6.28.NIT.06.item
ITEM_NAME="NITDroid 2.6.28.NIT.06 on mmcblk0p2"
ITEM_KERNEL=2.6.28.NIT.06
ITEM_MODULES=ext3 fat vfat
ITEM_DEVICE=${INT_CARD}p2
ITEM_FSTYPE=ext3
ITEM_FSOPTIONS=rw,noatime,errors=remount-ro,commit=3

~ $ cat /etc/multiboot.d/100-NITDroid-mmcblk0p2-noSD-2.6.28.NIT.06.item
ITEM_NAME="NITDroid 2.6.28.NIT.06 on mmcblk0p2 no SD"
ITEM_KERNEL=2.6.28.NIT.06
ITEM_MODULES=ext3 fat vfat
ITEM_DEVICE=mmcblk0p2
ITEM_FSTYPE=ext3
ITEM_FSOPTIONS=rw,noatime,errors=remount-ro,commit=3

~ $ cat /etc/multiboot.d/11-NITDroid-mmcblk1p2-2.6.28.NIT.06.item
ITEM_NAME="NITDroid 2.6.28.NIT.06 on mmcblk1p2"
ITEM_KERNEL=2.6.28.NIT.06
ITEM_MODULES=ext3
ITEM_DEVICE=${EXT_CARD}p2
ITEM_FSTYPE=ext3
ITEM_FSOPTIONS=rw,noatime,errors=remount-ro