Fascinating. Here in Singapore, I see at least one person on the train with a PSP everyday. Which gives me a vaguely-uncomfortable sense of moral superiority as I open up my DS...
There was one time when I was on a bus, and there's a young couple in the seats in front of me playing EBA over Wi-Fi, and another guy across the aisle with a PSP.
(By the way, how much time do you spend actually using PictoChat when you could be, you know, playing games? Yeah, me too. And you wonder why nobody dropped into the chat room? For stuff like PictoChat to work, it needs to be integrated into the DS in a more organic way, so you could e.g. get a little icon at the corner of your touchscreen while playing, to indicate that someone's also interested in chatting, and tap it to pause the game and launch chat.)
My overall conclusion is that handheld gamers are typically best found on public transport. That, and Singaporeans play a hella lot more handheld games than New Yorkers. (Although, this being the society that it is, it'll be a cold day in hell before a stranger comes up and says "that game looks cool". C'est la vie.)
Oh, and one more thing: don't discount the person tapping away at their mobile phone. Casual or not, I'd say they should be counted.