Wow. So many thoughts on this are running through my head.
First and foremost, I congratulate Robert and his encouraging and tech-savvy mother on their amazing accomplishment. Attempting Objective-C? Evaluating multiple third-party dev kits to pick the one best suited to the project? Four thousand lines in a new language? Sticking with a project until it's done and shipped? Those are all laudable.
Second, the journalism on this story has been poor as it spreads through the media. Different stories conflict on how much help Robert got from his mother. One report even switches back and forth between saying they worked together and calling the game a "solo effort"...in the same article. But the biggest problem is the title claim. Bubble Ball didn't unseat Angry Birds, it unseated a stripped-down Christmas-themed trial version, Angry Birds Seasons Free. Apple's free app market is volatile, and the Angry Birds suite has not been dominating it. The full, paid version of Angry Birds is still king of its hill -- #1 from spring to October of 2010, according to Wikipedia, and also #1 steadily for at least the last two and a half months. But two other free games climbed ahead of ABSF within three days of Bubble Ball doing so. And ABSF had only been there four days to begin with (source: the App Store and fingergaming.com [http://fingergaming.com/2011/01/17/top-iphone-game-apps-eas-the-game-of-life-returns-to-charts/]). Yet Angry Birds Free (no "Seasons") was released at the same time and has yet to break the top 10, and Angry Birds Lite, still another stripped-down trial version, has been around since April. I don't know its full history, but it oscillated between 7th and 9th from December to early January, yet is 67th as I write this one week later.
Finally, I worry that most of Robert's success is not because he made a really good game but because the company that makes the development toolkit he used promoted his app and because the media went into a frenzy. Look at the timeline:
Dec. 29: launch
Jan. 7: 0.3M downloads
Jan. 9: picked as App of the Week
Jan. 14: 1.5M downloads, reached #1 free spot
Jan. 18: 3.0M downloads
Marketing, not game design or production values, coincides with the big surges. The initial 300k in nine days, though... That's the part I wish I had a better handle on. More importantly, Robert and his mom need a good handle on it. If they don't know how they succeeded this time, it'll be harder to do again. Worse, Robert may let it go to his head and be mislead for years about how good he really is. (To be blunt, that's very good any way you slice it, but regardless, it's healthier and more advantageous for him to have an accurate self-assessment.)