at NV40 launch the Crytech guy refered to everything as "PS2.0 and PS3.0" was necessary..
Which leads us to belive that the "SM3.0 mod" is nothing but content update with SM3.0 support..
not SM3.0 exclusive.. you just dont want to run it on "todays" PS2.0 hardware..
Altho running it in PS2.0 on R420 is prolly very possible at similar rates and visuals as NV40 in PS3.0..
So its more a content update to use the overall speed improvments on the new generation.. seeminly..
Devs do most likly use both ATI and Nvidia hardware in development, if they fail to make their games run well on one of them, they will sell less games..
That is why Valve was annoyed at the problems with NV3x, they want everyone to be able to run their game well, and that means extra work for them if the hardware isnt doing that with standard implementations..