I don't have a smartphone yet, but I know the same thing will happen to me. In fact, that's the universal testimony: "I thought I didn't need it and now I can't live without it."
Have you ever thought about why? It's because this is the closest we've come to having our brains plugged directly into the intertubes 24/7. Information is a powerful drug. Getting used to having a smartphone changes YOU, even if only a little. Maybe it's ultimately for the good, but people should carefully weigh this before jumping on. I say if it helps you interact with the real world better and be more efficient in it, good. If it just makes you an even-more-distracted, angry-birds-playing information junkie, then not good.