That design has a couple of problems. Most of them involve preventing accidental scrolling while allowing intentional scrolling.
Walking can cause accidental scrolling. Have you ever tried to hold a pan of water while walking? It sloshes forward and backward because humans don't walk at a constant speed. Every step is deceleration followed by acceleration. Any vehicle changing speed can cause accidental scrolling. A car accelerating or braking, a bus making stops, a subway. Fine-tuning the code to detect when to scroll would be a nightmare. If you make it less sensitive, users will complain that they have to tilt the phone too far to scroll at a decent speed. If you make it more sensitive, users will complain that it scrolls when they're just holding it.
It's not as intuitive as touch scrolling. Tilt scrolling requires "zeroing" the phone and being very careful with how you orient it. Touch scrolling is simple: text on the screen moves with your finger. With touch scrolling you can flick your finger across the screen to start scrolling far/fast, then touch the screen to immediately stop scrolling. You could copy this behavior by allowing touch and tilt scrolling, but then it gets even less intuitive.
Yes, but you wouldn't have to use tilt scrolling in those conditions. Tilt-scrolling would be disabled by default. When the conditions were appropriate, you could opt to enable it simply by holding your finger on the screen for two seconds. (Touch scrolling would always be enabled.) Maybe a quick tap on the screen might turn off tilt-scrolling so that if your conditions changed, you could disable it before you lost your place.
If you were in a train that was accelerating, you could choose not to use the feature, or you could just use it while the ride was smooth and quickly switch to touch-only mode when necessary. For slight accelerations, you could also adjust for your environment's movement intuitively much like you naturally tilt a cup of coffee slightly as you move.
I think tilt-scrolling would be quite unusable, albeit a nice optional feature - would be nice to play with it to see if it really works. There is definitely a problem with scrolling on the iPhone when it comes to long pages - making scrolling to the bottom of a page pracically impossible in some scenarios. There is this bias towards scrolling to the top, but not the bottom embedded into iPhone's design. That's why I made the End Of Page app for Safari on the iPhone - it lets you scroll to the bottom of very long pages without having to flick forever. It is free - I am releasing it in the hope that others find it useful. I should be on the App Store on Friday, provided all goes well with the approval process.
Walking can cause accidental scrolling. Have you ever tried to hold a pan of water while walking? It sloshes forward and backward because humans don't walk at a constant speed. Every step is deceleration followed by acceleration. Any vehicle changing speed can cause accidental scrolling. A car accelerating or braking, a bus making stops, a subway. Fine-tuning the code to detect when to scroll would be a nightmare. If you make it less sensitive, users will complain that they have to tilt the phone too far to scroll at a decent speed. If you make it more sensitive, users will complain that it scrolls when they're just holding it.
It's not as intuitive as touch scrolling. Tilt scrolling requires "zeroing" the phone and being very careful with how you orient it. Touch scrolling is simple: text on the screen moves with your finger. With touch scrolling you can flick your finger across the screen to start scrolling far/fast, then touch the screen to immediately stop scrolling. You could copy this behavior by allowing touch and tilt scrolling, but then it gets even less intuitive.