Well I know with desktop RAM, DDR2 has more pins than DDR... I think DDR2 will fit in a DDR slot, BUT won't work. They are two different standards, running at different speeds, different voltages, etc etc...
If I held one of each up and you looked at them you wouldn't be able to see the difference, you have to have them literally overlapping so you can see the exact positioning of the key to be able to tell, it's so similar.