My understanding is that wort gravity does not affect hop utilization. Instead, a larger boil volume will increase hop utilization, while a higher amount of break material will lower hop utilization. This would mean that for all grain batches, high wort gravity is correlated to low hop utilization, but for extract batches it would be irrelevant.
Of course, this contradicts a number of brewing resources, such as Palmer's How to Brew and multiple articles in BYO. I've read that Palmer has since changed his mind (but I have no good reference for this). I've also read that BYO and Basic Brewing Radio did an experiment with extract that determined boil size, not wort gravity, was the contributing factor to hop utilization (again, can't find the reference).
So my question is this: Does wort gravity affect hop utilization? In particular, I'm looking for a solid reference (maybe even with a solid experiment) rather than general brewing knowledge or forum posts.