Hey. Thought I'd chime in since I'm a film maker too. Have worked at Pinewood on several films but more importantly to this conversation, I love watching films.
There are two arguments to consider here:
As film makers we want the film we make to be viewed with complete and utter attention. Every frame is there for a reason. That's why film makers believe the cinema/IMAX screens are better...no distractions (you get told to turn your phones off in a cinema, so no distractions is definitely the aim here). No distractions means we're more able to be immersed into the story.
As an audience member, I say that any film worth its weight in gold will pull my attention to it no matter what size screen it's on.
Film has, for years, been made to work on big cinema screens. Some films just don't call for it. Avatar, for example, does not work very well on a smaller screen (even on a 50" HD plasma TV with a blu-ray disk player). However, films that are less visually intensive work very well (comedies and such). It all depends how watertight the film is.
Pixar are a great example of this. Sure, it's lovely to see their films on a giant screen but they're so watertight story-wise etc that they work just fine on smaller screens. Some films I wouldn't dream of watching on a smaller screen, others wouldn't bother me at all (the biggest TV in our house as I grew up was 14" and we all gathered around it every Sunday to watch The X-Files no problem. Now, we feel a bit silly doing something like that. People get a little spoilt with technology sometimes I think, and that does contribute to arguments such as these (what size screen you are used to etc.)
For that reason, my vote goes into 'definitely not retarded to watch a film on your iPhone BUT it depends on the film and what you have available'. Cinema is absolutely the best way to watch a film, I don't think that can be argued. But it's not the ONLY valid way to watch a film and I think many film makers would do well to remember that.
I'd like to ask Lynch and Cameron what they'd prefer: someone not watching their film at all or doing so by their only means (which could an an iPhone screen, or an oooooold tiny black and white TV, or a laptop etc.)? I think they'd want people to watch it no matter how they did so. After all, film makers tell their stories to be enjoyed by others - that means accepting how other people enjoy them no matter what.