I thought about this thread the other day as something similar happened to me a week ago. I was assigned a middle seat on a European flight but oddly enough there was nobody on the aisle or window as we were almost finished boarding. Before the door closed a very attractive woman sat in the aisle and as I was contemplating moving to the window she asked me to put her carry on under the window seat. We were thus going to be aisle and middle for the duration. We soon got to talking, found that we were in similar businesses, had common interests, etc. After about an hour of chatting away she did something odd - I had my arm on the armrest and she simply draped her arm on mine in a very intimate way, much like you would unconsciously do with your significant other. It wasn't really a come on even, it seems like she just felt a connection and wanted to hold me for awhile. After we landed we went our separate ways and that was that.

So I guess I can confirm that this sort of thing does happen, but I wouldn't ever expect it again.