Are there any other "hidden" bits of dialogue like this in Portal 2?
Some hidden dialog is triggered when you get the "Good Listener" achievement. Then, at the end of the game, each of the cores you pick up have loads of dialog. The final turret dialog can also be considered hidden, since it requires translation.
Wheatley has tons to say, if you hang around long enough to listen to him. Sometimes you just have to wait around, and other times you need to interact with the environment.
Some of the better easter eggs, however, don't necessarily contain dialog, but play weird sounds or show you secret places.
Most of the sound files from the game have been uploaded to YouTube by various people:
- All quotes from GlaDOS (parts 1, 2, 3, 4, 5, 6)
- All(?) quotes from Wheatley (parts 1, 2, 3, 4, 5)
- All quotes from Cave Johnson
- All quotes from defective turrets
- All quotes from normal turrets
- All quotes from Prometheus ("different") turret
- All quotes from Fact Sphere
- All quotes from Adventure Sphere
- All quotes from Space Sphere
See Steam\steamapps\common\portal 2\portal2\resource\subtitles_english.txt for all dialogs in the game, there are quite a lot of them and their corresponding sound file names are listed.