Can someone with more experience chime in and explain whether Photoshop on a tablet is really useful?
Several of the photo manipulation apps on the iPad already do this and it's far superior to selecting brush widths, to say nothing of obviating the precision problem [2].
In short, the screen size of the iPad is more a roadblock to its being a replacement for desktop photoshop. Though its portability, comfort and the ability to focus would be a huge advantage for anything in the sketch/draft/mockup stages of work.
And the lack of (true) pressure sensitivity makes the iPad a non-starter as a wacom-replacement. I suspect this could be solved by a bluetooth stylus. But the iPad alone is simply insufficient for many wacom tablet uses.
[1] e.g. the 'brush' always applies the selected effect to the area of the image under your finger. If you wanted more fine-grain accuracy, you zoom the entire image, so that the number of image pixels covered by your finger is reduced.
[2] Which, at a fixed resolution, absolutely was and is a problem.
This is not the same as photo manipulation but it shows that tablets have some excellent artistic uses. Painting with Brushes feels very fluid, even if you are a beginner. More professional tools are not quite so accessible (and not only because of their price).
What will we do with tablets? I have no idea. We are at the very beginning. I was just playing around with GarageBand and it is now at least clear to me that software on tablets is different and can be wonderful. Some things will not work as well, some will work so much better and the only way to find out is to try.
I think an iPad version of photoshop could work as a simple photo retouching app as shown in this demo. But it will be really, really hard for Adobe to get the same level of functionality into an iPad app that they currently have on the desktop. Can you imagine using the type tool, or manipulating a document with hundreds of layers and groups on a 10" screen? Seems difficult.
That said, a fast tablet with video out to a bigger screen would be pretty amazing, actually: palettes and details on the tablet, image you're manipulating on the big screen.
The iPad has a long way to come in equaling the speed of desktop, but you can almost see it down the road.