The Bespin thing is pretty impressive, I never noticed that before, either.
As for the cave.. the old EU claimed that Yoda had killed a Dark Jedi there, but of course that was always non-canon. It's also questionable as to how much of the truth Yoda tells his students for their tests. It's definitely quite possible, probable even, that the cave was just a plain ol' cave.
On the one had, it's "strong with the dark side", on the other, it contains "only what you take with you"... the dark side he was referring to probably wasn't in the cave at all, it was in Luke. That was the whole lesson of the cave, after all.. to demonstrate the darkness within that the student must learn to accept and control. Tell a student that there's something scary in there, and their own fears and doubts will fill in the rest. It wasn't until RoTJ that Luke stopped being afraid of Vader. He overcame that fear quite well in ESB, but Luke was probably terrified during that entire encounter.
Good lord, ESB is a good film.
I think the only thing that would make make it a PERFECT film would be a line like this: "Bespin? Hmm. It's gonna be a long journey but I think we have enough fuel and food to get there"
Luke felt something wrong. He said he felt cold. Then he asked what was in the cave.
Luke may have brought his experience into the cave with him, but something about that cave was messing him up before he ever went into it.
Yeah I know, Yoda could have made Luke feel darkness there. But nothing about the scene leans that way. It plays like the cave just had some pre-existing bad hoodoo.
So if this Bespin theory is true, then we must assume that the film crew were building the cave that way on purpose. I can't believe this has never been mentioned before. I've read pretty much everything on the making of this movie that there is, I've never heard anything about it.
I'm willing to bet that this Bespin theory is true--hard to believe it has never been talked about before....wow....
IIRC Dagobah was the last shot for ESB as Bespin and Hoth had already been filmed because the Yoda puppet took so long to finish.