Okay, so onto ESB.
Looking at the timeline (http://originaltrilogy.com/forum/topic.cfm/Original-Trilogy-Times-Chart/post/411140/#TopicPost411140), it's really easy to subdivide this movie into three equal-ish parts:
1. Hoth
2. Dagobah/Asteroid Field
3. Cloud City
These may or may not represent the acts of the movie, though. Let's see:
The "problem" of the movie, I would say, is that Darth Vader is looking for Luke Skywalker. Which is sort of odd, because right there, it makes it seem that Vader is the main character of the movie... which isn't true from a scene-by-scene analysis of the movie. But if you think of the "problem" that drives the plot of the 3 acts, I think that's it. Is there a better one?
Act 1- The fact that Vader is looking for Luke is introduced in the opening crawl, just like the Death Star problem was introduced in ANH's opening crawl. Vader is looking for the Rebels because they have Luke. The problem isn't very serious, however, because we all expect the Rebels to escape Hoth. Act one is over when the good guys get their minor victory and everyone escapes.
Act 2- Obviously, Luke on Dagobah is the major setpiece in this timeframe, but it doesn't actually have much to do with the 2nd Act of Empire Strikes Back. Instead, it's really the middle of the Macro (Trilogy) Act 2, and Luke is setting about solving the Macro problem (The Evil Empire) and not the immediate movie problem (Vader searches for Luke) except that he is hiding. For this reason, Luke must leave the safety of Dagobah to end Act 2, and get Vader's quest back on track. Meanwhile, Han and Leia are also working on the middle of the 2nd Macro act and solving the macro sub-problem of the love triangle. Yes, Vader is chasing them to solve his main problem, but that is not what we spend the majority of our time watching. Vader hiring Bounty Hunters though, increases the threat that Han and Leia will get caught, which drives them out of the Act 2 stalemate.
Act 3 - I'm not sure which scene exactly starts Act 3, but it's either at the very start of the dark atmosphere on Cloud City or it's Dinner with Vader. Usually we see the hero prevail in Act 3, against all odds, and it seems to be true in this case, except again that Vader is the "hero" of Empire's 3 Acts. He captures the wily Han Solo, and successfully draws young Skywalker to him. They fight, and the first time viewer expects a clean win on Luke's part, but perhaps they haven't been paying attention to the actual structure of the movie (who is, when you have the great sabre battle going on?). Leia and company escape, but that's not the focus of this story. Their part in "the problem" has been played out, and they're just setting up their ability to participate in the sequel- the macro 3rd Act. The 'twist' ending is of the variety where we reinterpret what the problem actually was. We thought Vader was searching for Luke because he is his enemy, and possibly because he wants to turn him to the Dark Side, so his real motivations surprise us. He wants to turn him to the Dark Side because he is his son, and he wants to overthrow the Emperor.
At any rate, Luke's defeat and subsequent escape is the end of Empire's 3rd Act. That and the few minutes that follow form the perfect ending to the Macro 2nd Act, as the Heroes are at their lowest, most unlikely to win scenario.
