I've spent a large portion of the last week lugging my 3DS around wherever I go. Not because of the stunning quality of the games, you understand; truth be told, I'm not especially bothered about any of the launch titles.
No, the root cause of my minor obsession is the bizarrely adorable Streetpass concept, where passing Miis migrate to your system and help rescue you from captivity. It's silly, largely pointless, and one of the most charming ideas I've ever seen.
On that note, then, I can't imagine it'll be long before we start seeing similar social gaming ideas integrated into mobile phones. I've only been wandering round London for a week and have already attracted 12 Miis into my 3DS. Imagine how many iPhones and Androids you'd wander past unwittingly in an average day.