With all due respect, live tree branches absorb and retain heat very very differently than rocks and minerals. While it's true that they do absorb some, and do become somewhat warm, it's not going to become much warmer than the surrounding air. The iguana will still heat evenly. The biggest issue here is that the belly shouldn't be warmed faster or to a higher degree than the back, as stated in the articles I posted earlier. Not to mention that you can get burned by touching a tree. And you can't prevent the ambient temperature from warming other surfaces... that's a given... but you can control which surfaces are in the enclosure to warm in the first place. I'm just trying to suggest that rocks, with the potential to heat up to high temperatures and retain warmth very very easily, might not be the best choice.
While I can repsect that feral iguanas in Florida may bask on flat rocks and sand, asphalt and metal roofs, this is hardly the norm. I don't think we can compare how a species should be getting it's warmth to feral populations. Iguanas survive in Florida because the humidity and temperature meet their needs. Florida is much more densely populated than, say, the middle of a rainforest in Belize. And anyone who has walked barefoot on a beach in the middle of a summer day knows how dangerously hot the sand can get... let alone the asphalt, roofs, and rocks. Temperatures that high on a surface are comparable to a malfunctioning heat rock, and I don't think that's something any of us would purposely expose our igs to. So why do they bask in these areas? Well, why do they lie on hot rocks until their insides cook and their skin burns?
I think in the next 10 or 20 years we'll start to see a slow evolution in the feral iguanas living in the southern US. I've never been to Florida, but I'm not aware of any tropical rainforests down there. If green iguanas are evolved to live in the trees in the rainforests, they'll have to evolve differently to live on the ground in urban Florida. No matter how well they seem to be doing now, they do better in their native homes. If you had to release your iguana(s) into the wild right now, would you prefer to do it in, say, Costa Rica, or Florida?
So let's talk about their natural environment. Rainforests consist of huge trees that create a tightly woven canopy that lets in very little light. If you live at the bottom of that canopy, it's warm and it's humid, but there is little to no direct sunlight - depending on the age and type of rainforest you're in. In an older rainforest, there is very little ground cover because the plants can't get enough light to grow. The higher in the trees you go, the more natural sunlight you get. There's no asphalt or beach sand, and no metal roofs to sit out in direct sunlight all day and absorb the sun. In the areas that are populated by people, there's little development and these things don't really exist, anyhow - not in great numbers, anyway. You just don't go to the rainforest (or watch a nature show) and see groups of iguanas lying around on rocks on the rainforest floor. They're up in the trees trying to get closer to the sunlight, and eating and basking and doing iguana things.
So maybe iguanas evolved as tree dwellers because, in their environment, it's the best way to get close to the sun. And for folivores, the best pickings are in the trees. It just doesn't make sense to come down for any reason other than to lay eggs.
I can appreciate the rock cliff climbers argument, though. I think it would be an amusing sight to see... a "Mission Impossible" iguana scaling an impassable rock cliff 
Do you, by any chance, have a picture?
While other reptiles may greatly benefit from contact heat, it's important to recognize that most other reptiles, with the exception of a few more arboreal lizards and snakes (tree boas, anoles, etc.) are terrestial in nature. I still think that iguanas can benefit most from an environment that is based *mostly* (I can't say soley, since, as you said, you can't prevent ambient heat from heating surfaces-I just don't think this should be done on purpose) on ambient heat.
I think it's important that their bodies are allowed to heat evenly. If the temperature of a surface they're lying on is greater than the ambient temperature, their bellies will heat up faster than their backs. If the temperature varies greatly, their bellies will heat up much faster than their backs. That just doesn't seem right...
Of course I understand that our iguanas are not in the wild and there's a whole lot of things we just can't provide for them. So maybe comparing iguana keeping to the life of an indemic, wild iguana is unfair and, ultimately, null and void as discussion material.
As a side note, I'm going to clarify, of course, that I'm talking about green iguanas only. I know desert iguanas bask on rocks, as do marine iguanas and other species.
Regards,
Alika
