Actually, from what I have read the 1X slots will probably get peripherals first. Think Gigabit ethernet and sound cards. Yeah! For a 4X slot, think dual and quad Gigabit ethernet cards or high-end RAID cards, or A/V editing. There is already a 1X Gigabit ethernet card based on 1X PCI-E reportedly for sale, but I have yet to find it.
Edited to say: I don't know about backward compatibility between slots. There is not enough data yet and this side of PC hardware is just starting to develop. We will see.