I always thought that both armrests belong to the person in the middle seat, but it depends on who you ask, according to this article on air travel etiquette in the Wall Street Journal. The article also tackles:
• What to do if the knees of the tall guy next to to you splay into your space (a frequent flier says, "Drop something on the floor. When he hopefully picks it up, reclaim your legroom space." Snark.)
• How to handle it if the people you have to climb over to get to the restroom are snoozing (I agree with the majority, you have to politely wake them when nature calls, though I like the idea of the aisle person asking every 90 minutes or so if anyone needs to get up. When I'm in the aisle seat, I get up pretty regularly so it's generally not a problem for my seatmates)
• What to do about rowdy kids (love this response: "I watched one flight attendant handle this adroitly by saying she 'would hate to have to put him off the plane.' Not another kick." But I generally try to ignore it if at all possible. One thing I learned the hard way: Never play peekaboo with a toddler sitting in front of you unless you want to continue playing peekaboo for the next three hours.)
• Dealing with the stinky-food syndrome (says an ethics professor, "Airlines have brought this on themselves by eliminating food service. Not only did I have a middle seat [recently], I was in the back and all the food-for-sale was gone by the time they reached me. I got out my smelly cheese and ate it in front of my seatmates." I think I sat next to him on that flight!)
• Whether or not it's OK to recline your seat (whether it's ethically OK or not, I try not to now that seat space has gotten so small).
I had a weird one on a flight recently where the guy behind me put his feet so far under my seat that they came out on my side. Then he reprimanded me for knocking into them—like I knew they were there! It was a short flight and he seemed pretty testy, so I just tried to avoid his feet in my space, but I'm pretty sure he was in the wrong on that one.
When in doubt, I think it's wise to err on the side of kindness. After all, we are stuck with each other for the duration. Might as well make the best of it.