Yes, dd works fine for dumping /dev/hda3 and then restoring it later. I did this during my migration from my Mark 2 to the Mark 2a that I bought in the fire sale.

Not sure why it's not in the FAQ currently. For the FAQ master to add:

To backup the data: (Asuming drives are in RW mode for this)
dd if=/dev/hda3 of=/drive0/filenametobackupto

To restore: (drives do not need to be in RW mode)
dd if=/drive0/filenametobackupto of=/dev/hda3