[note: in 2009 versions I don't experience this issue, though it is still technically possible - Steve 09/2008]
I run into this every now and again. Revit user complains that all the model objects are off in a view or all the annotation...or imported files. They've tried all the usual suspects like Visibility/Graphics, Worksets, Phase/Phase Filters, Filters, Design Options and so on. They just didn't notice that every category in the Visibility/Graphics dialog (under one of the tabs) was gray and not selectable.
The question is how did it happen, surely it wasn't done purposely. Enter the good old Space Bar, pun intended. AutoCAD/ADT users are so used to tapping the space bar to start a command that they are hardly aware that they are still tapping it from time to time in Revit, that and Revit uses it for flipping or rotating some objects.
The trouble is that opening the Visibility Graphics dialog and immediately hitting the space bar unchecks the top most check box next to: "Show (Model,Annotation,Imported) Categories in this view". G That's what Windows assigns to the Space Bar for keyboard behavior when Window's focus is on a check box.
So don't let the Space Bar getcha!