I also have a problem not able to join islands now that I've upgraded... I've retained both the world folder, and the local folder. That islands.ser file looks like it *should* contain the island name/coords mappings, but it doesn't seem to work for me. I went from 1.0.2 up to 1.1.0, and now I'm stuck on one island and cant get to the other one I created...
At this rate I'm going to have to go back to 1.0.2 and see if that fixes the issue, and then just never upgrade for fear of losing access to other islands!
Any ideas anyone? This is a real pain....