You don't need resolution independence to support different screen sizes. Thank about that, traditional PCs and Mac applications have been doing that for years. Making your application work on multiple screen sizes and true resolution independence are two related but different things. Developers could be doing the former right now with no new Apple technology, and they should be planning their apps from the start for dealing with different resolutions. If we ever get system-wide true resolution independence, that's great, but it's not necessary.
Even if you change nothing, increased resolution should provide higher-quality antialiasing for text at the same viewing size (particularly with apps using a lot of text drawn into Core Animation layers, since the iPhone and Core Animation don't support subpixel antialiasing). 720x480 would be perfect, and while I don't think we'll see it, I wouldn't mind the screen also being 10%-20% physically larger (much of which could be taken from the current faceplate without increasing the device dimensions much. 720x480 is about 2.25x the current amount of iPhone pixels, and that would make a huge difference on the amount of things you could do on the device. That's getting into the range where you have real estate for serious things if you wanted to (spreadsheets, coding, more complex games such as RTS).
Even if you change nothing, increased resolution should provide higher-quality antialiasing for text at the same viewing size (particularly with apps using a lot of text drawn into Core Animation layers, since the iPhone and Core Animation don't support subpixel antialiasing). 720x480 would be perfect, and while I don't think we'll see it, I wouldn't mind the screen also being 10%-20% physically larger (much of which could be taken from the current faceplate without increasing the device dimensions much. 720x480 is about 2.25x the current amount of iPhone pixels, and that would make a huge difference on the amount of things you could do on the device. That's getting into the range where you have real estate for serious things if you wanted to (spreadsheets, coding, more complex games such as RTS).