Katie,
No, I didn't have the 3d selection tool, or anything 3d going at the time. I had taken some pains to make sure that this entire drawing was strictly in 2d, given my experience earlier (I have to ship a drawing, not learn 3d right now!)
About the only thing that I can think of that might have caused this condition was that prior I had grouped then duplicated/moved/rotated some guides, then ungrouped them. Before the ungroup command was executed, I was presented with a mysterious dialog, something like, "release objects with class information? yes always, never, just this time" (or something like that). I don't remember my response, mainly because I didn't understand the context of the dialog question.
About then is when the "iso-cube" trouble seemed to start. Thankfully the other contributors nailed the cure.
andy