I have designed about 100 or so PCs in various game systems on my Palm (which is slowly dying), and I run them off of it with no problems- scrolling up or down takes about as much time as leafing through pages. I anticipate doing so with my iTouch eventually as well.
Call me old-fashioned, but I like to see most of the pertinent details without needing to scroll, so I like having something close to an 8x11 size.
Ultimately, though, the character sheet isn't the interesting bit - while some might consider a character builder really cool, display of the sheet is trivially easy to do on paper, even with the builder. To use the software business vernacular, sheet display during play isn't a "pain point". Technophiles may pick it up because it is seems sexy, but you're not really saving the player much effort.
Die rolling - I've had the opportunity to compare in my Deadlands game (in which the dice are somewhat more complicated than D&D). Two players are using devices for die rollers, and the others use physical dice. Consistently, the people using physical dice get it done more quickly. No savings there, either.
Integrate with a virtual tabletop, so you don't need a battle-map and figures, and now you're talking relieving a pain point. You're also talking about far more development time, though.