There's a few problems with that. First, Meatball provided this list for us:
Nexus One: 3.7" 800 x 480
HTC HD2: 4.3" 800x480
Nokia N900: 3.5" 800 x 480
iPhone: 3.5" 480 x 320
Now to exactly pixel-double an iPhone app, we'd have to multiply the current screen rez by two, which equals 960x640. I've never heard of an LCD being manufactured in that specific size. None of the other phones on the list above run at that rez, nor do any I've heard of. My understanding of LCD manufacturing is that they come in size and density "steps". I'm sure they can customize for big orders, but to what extent and at what cost I'm not sure.
If Apple tries to come up with an intermediate size screen, then they can't just pixel double old apps. They have to do interpolation, and it will look ugly and unbalanced.
All this seems too much of a pain for moderate benefit and many drawbacks. The benefit of course is sharper graphics and more complex screens, but the finger buttons can't be any smaller than a finger allows anyway.
Drawbacks include higher hardware cost, lower battery life, higher demands on the graphics processor, and another type of screen for app developers to optimize for.
I'm still having a hard time imagining it will happen this year.