Well, it looks like everyone's had a great time. Looks like it's time for my two bits.
It seems to me that the reason that passenger cars don't have V-8's is because V-8's generally are less reliable, more expensive, heavier, take up more volume, have less fuel efficiency, and V-6's generally don't. (did I forget anything?)
So the question is "is this true (generally)?"
1. Less reliable: I would say that comparing two engines, a V-6 and a V-8, each getting similar horsepower, the V-8 would probably be more reliable because it is a lower RPM engine, due to it's larger displacement. Lower RPM, less wear and tear on the engine. However, the counterpoint is that the V-8 has more moving parts, and therefore is less reliable. However, I think I heard somewhere that a OHC V-6 actually has more moving parts than a OHV V-8. I may be wrong.
2. More Expensive (to make): Somebody said that it's dirt cheap to make a Hemi compared to an OHC V-6. I don't know why that would be, but that may be right. As for building a low tech V-8 vs a high tech V-6 with similar horsepower, it would seem that the V-8 would be cheaper. Also, GM has been making small-blocks for so long that it would seem that they could build them very cheaply, at least compared to new high-tech V-6's.
3. Heavier: New aluminum-block V-8's are said to be very light. I don't know how they compare to V-6's, but I would assume V-8's to be slightly heavier, as long as they are significantly larger displacement than the V-6's. If the V-8 and the V-6 have the same displacement, than it would depend on engineering issues. Personally, I don't think you can generalize V-8's and V-6's in this catagory.
4. Take up more Volume: OHV V-8's are smaller volume-wise than OHC V-6's, due to packaging issues. This is only has to do with OHV vs OHC, not V-8 vs V-6. OHV V-8's are bigger than OHV V-6's, which apparently only GM has.
5. Less Fuel Efficiency: V-8's are generally less fuel efficient because they have bigger displacement, plain and simple. However, many V-8's are quite fuel efficient as seen in comparison to a V-6 in a Power:MPG ratio. Compare a 400hp Vette getting 20mpg compared to a 250hp Sedan getting 25mpg. Most people don't see it that way, though.
It seems to me that the fundamental differences between V-6's and V-8's getting similar horsepower is price and fuel efficiency, with price going in V-8's favor, and mph in the V-6's favor.
Most everyone appreciates driving a car with an engine that has lots of power on demand. V-8's are the cheapest way to get lots of power.
So, if V-8's are ever to be sold in high-volumes, an engine must be built that can compete with the V-6 in fuel economy while still providing cheaper power. This may be done through DoD, Hybrid Transmissions, 3-valve heads, efficient design, etc.
It seems to me that a cheap 300 hp V-8 that could get 24 mpg combined could find many applications in passenger cars, especially if it's RWD. Also, especially in larger cars, the additional torque supplied by V-8's will always be a plus.
So, if you haven't already guessed, I hope that V-8's do NOT decline in availiability in the future.