I read somewhere recently that, during one playthrough, the player noticed that his character reacted to the cold when first going outside. Do you think this will have an effect when you are high up in one of the mountains (ie. will the colder temperatures negatively effect the player character?)

Also, does anyone know if there is going to be a hardcoe survival mode as per Fallout NV (must eat food and sleep in order to survive + killed more easily during combat)

There's no hardcoe mode.

In the recently released demo video, all of the food items seem to restore your health. I thought that this might be useful to incorporate into some kind of survival mode along with having to set up camp out in the middle of nowhere to rest in order to sustain your adventuring. (only to be awoken by a sabre-toothed tiger?)

hardcoe mode aside, I think it'd be cool if your character reacted to the environment around him, something like Morrowind when there would be those blight storms, and your character would walk while covering his face...

It would be amazing I think, but they've said it won't be in. :sadvaultboy:
As it is, having the PC (and NPCs?) reacting to the weather is great, even if it's just on a superficial/animation level, and won't affect the stats at all.

Is this confirmed - if so, do you have a link?

It was confirmed long ago - no hardcoe mode. Sorry

But what about the PC reacting to the environment?