And in case you’re wondering, unchecking the first checkbox reveals a top-level “PDT Updates” node with no children.
I love Eclipse when it works. However, every once in a while things like this will happen or random Exceptions will get thrown, and I’m left wondering if perhaps computers are non-deterministic.

I’ve been having this same problem. It seems to only be related to PDT, which leads me to believe they have an issue in a config file or something. Worse yet, when you go to Help -> Software Updates -> Manage Configuration, choose PDT and Find Updates, it doesn’t show the invisible entries.
Also, oddly enough, the number of invisible entries seems to grow slowly over time. Mine just went from 9 to 10 this morning.