I gotta go with Marcus here.

Maybe MB set the bar too high for themselves when they introduced the S550. Maybe they should've introduced the S450 first and then bring on the S550 a year later (like they did with the GL).

On the streets of New York, I can tell you that there are more W220 S430's out there than there are W220 S500's, especially for customers that use them as company cars or limos. Why should they put down more money for a bigger V8 (500) when the 430 was sufficient? I just don't see the reason for having two V8's in the same class.
I think MB learnt their lesson here and that's probably the reason why we don't have an S450 in the States.

An S350 wouldn't work here either. I've only seen one or two W220 S350, but they were only made available during the last year of the W220 and it had an old engine/tranny.