But then again...there were a lot of context clues...
Indeed. I began to suspect the outcome early on, but nothing concrete enough to be 100% certain. They really did follow the guideline for a great character revelation - they fed us just enough information to keep us on the edge of understanding, and then blew us away with the end result. Well, maybe not blew us away, but the way everything came together at the pinnacle of hints was superbly crafted.
In all honesty, that was the only way Sakurai would have gotten me to like R.O.B. the way I do. He still isn't my favorite character, but I do look at him a lot differently now! He isn't just some lame, brainless, and mindlessly added character after all!