They can be, depending on how good or bad your toolkit is.

Desktop OSes usually also have displays much larger than the windows of most applications, and UIs that do (somewhat) resize when shrunk/grown.

You'll notice that the problem today is a constraint on the maximum resolution of displays, and then you'll see the solutions of the mobile world impact desktop computing (DPI-sensitive interface scaling.)

Apple is bringing the advantage that video game consoles have, namely that a given line of hardware is the same from the software's perspective across all devices. All iPhone 4 devices have the same minimum internal memory, same display, same communications features, etc.

The "disjoint" nature comes from the wildly different hardware subsets, over which Google has no control. Froyo being made to run on everything from weak ARMv6 chips from 3 years ago to bleeding edge Cortex A9 chips, with various and sundry additional hardware makes for a confusing array of devices.

I will agree that it has no real impact in the long run, so long as the hardware companies focus on delivering performance. People who make these predictions are basing it on the abysmal spread of Windows Mobile devices, which had a confusingly large array of devices with strange screen shapes, some being touchscreen and some not, and a UI toolkit that completely failed to scale with screen size.

There are tools in place in most OSes now that you can specify the DPI of your screen and widgets will shrink/grow appropriately. So you end up with huge UI elements in the exact same spot regardless of screen resolution. I don't think anything makes use of it (see the iPad doing silly stretching of app windows.)

If anything, the Tab's higher resolution will force Google to get resolution independent UI rendering working 100%, just as I hope MeeGo tablets and phones force the same for Linux. There's no reason to be stuck to bitmapped UI elements these days, on any OS.