I thought about that a lot before my last laptop upgrade. If I hadn't been buying a new laptop, I would have done it; but with a new laptop at play, I decided a MacBook Air would have roughly the same weight and far better ergonomics than an iPad with a keyboard.
Actually, decoupling the display from the keyboard (iPad with BT KB) could result in a much more ergonomic setup. But in all honesty, the only time I use an iPad for dev is when my air is charging. Even Textastic pales in comparison to a hard keyboard/mouse.
