IPv6 is more like "solar power" in your analogy. We're going to run out of oil/IPv4 someday and have to replace it with something, but not necessarily IPv6.
Actually IPv6 is probably more like "atomic fusion" - some utopian grand project that will fix everything but nobody quite knows how we're going to get it to work.
The idea that IPv4 will be replaced with something that isn't IPv6 is no longer credible. We need a solution in 2012, and nothing else is going to be ready by that time.
Well it wouldn't take much for something else to outpace the adoption rate of IPv6. I think something will come along that is backwards compatible with v4. We're already seeing things like carrier grade NAT that are just bandaids but we can live with bandaids for a long time...
Actually IPv6 is probably more like "atomic fusion" - some utopian grand project that will fix everything but nobody quite knows how we're going to get it to work.