Will scaled-up iPhone apps be a usability nightmare on the iPad?

Steve Jobs told us last week that most iPhone apps should work on the iPad right out of the box, reassuring users that there will be a huge base of apps for owners to tap into right from launch day. But how well will these apps work scaled up from a 3.5-inch screen up to a 9.7-inch screen?

This thought came to me earlier today after reading Ed Bott's latest piece on apps for the tablet PC:

So what Apple is doing right with the iPad is insisting that the only apps you’ll be able to install will be those that are designed from Day 1 with full multi-touch support, either for the iPad itself, as Apple is doing with its base software package, or for the iPhone and iPod Touch.

This got me thinking about the third-party apps that I use on my iPod touch and how well these might work scaled up. I'm not a huge "app rat" but I use my Twitter clients (Twitterific and TweetDeck) often, and have a small handful of games installed that I turn to when there's nothing else to do, but experience with these gives me an insight into problems that iPad users might face.

One problem I see is that while on the iPhone/iPod touch, you're controlling most of thee apps with the flick of a finger or thumb (I'm pretty adept at using my iPod touch one-handed), on the iPad this is going to translate into quite a big hand gesture. Not only are larger hand gestures less precise, they're also more tiring on the wrist, especially over long periods.

Another problem is to do with games. A lot of the games assume that you can use your thumbs on the side of the screen to interact with the on-screen controls. The iPhone is only 4.5 x 2.4 inches, but the iPad is much bigger, at 9.56 x 7.47 inches, and weighs in at 1.5 pounds. At those dimensions these thumb-driven actions become a lot trickier. In fact, on games such as Resident Evil 4, I just can't see how the UI will work on the larger tablet.

I also think that scaling is going to be plain crazy for some apps. Imagine a calculator or notes or fart app spread across a 9.7-inch screen ... it just doesn't feel right, does it? Another problem is going to be fonts. I can see the text on some apps being simply huge.

While I can see why Apple has decided to stick with one platform (for now), I do think that apps are going to be a problem and that developers are going to have to put the legwork into fixing them up so the UI works on the iPad. Otherwise there's going to be a lot of unhappy people out there.

Apps running, and apps being usable are two very different things.

So, iPhone/iPod touch users, whaddya think?