Joe: Facebook is in a category of being so ubiquitous that I don't feel like it needs to have its own crazy, unique style that a lot of other apps could maybe justify. I thought it was better not to go crazy and reinvent the wheel. It was important to make it feel, not so much that Apple made it, but that it was following the same style guidelines that Apple established in its built-in apps.
The photo viewer in Facebook was probably the part that took the most effort in that way. Apple's built-in Photos app is just amazing, and I was very disappointed that more of that code wasn't available in the SDK [the iPhone's code toolkit]. I had to build my own, and I still don't feel like I nailed the physics for it. Getting the photo viewer to work with a similar motion as Apple's took a lot of experimentation. For example, when you zoom in as far as you can go, it lets you zoom just a little bit past the limit, and then when you let go, it bounces back. That takes a lot of time and attention, but those details are really important to get right.
With the touchscreen, you can give users the illusion that they're touching a real thing, but if it doesn't move like the real thing, like it jitters or jumps around, it breaks the illusion. You feel like you're using a computer again and not just moving something physical around with your finger.
The touchscreen makes all the difference in how an interface should be designed. On a PC, it's okay for things to be a little digitized because the whole thing is an abstraction anyway: using a mouse is almost like using a robotic arm where you're controlling this prosthetic. There's a distance between you and the interface. But when the robotic arm is gone and you're really touching the screen, things should feel more realistic. You have to design for that.