Whenever I'm on the subway (or any other form of public transit, really) I almost always have a book with me. Maybe it's because I'm conscious of being a reader, but I always seem to notice other people who read on transit. And usually I'm curious about what they're reading and how they came across the book; when it's something I've read, I always have to suppress the urge to ask them what part they're reading.
Apparently, I'm not alone. The Missed Connections blog has frequent posts about someone who noticed someone reading and was intrigued enough to try and find them. I especially like this one, about a missed connection in a library, and this one, about a beautiful commuter reading Catch-22.