First one: which size?
Currently every iDevice except the iPad has a 480:320 screen res. Hence the 200k+ apps in the app store are designed for it. So if they put in a new screen res, those 200k+ apps will look awful on the new iPhone, UNLESS, it is an exact multiple of the old resolution, like 960:640. It looks like as the 4G has an A4 in it, so there should be no problem handling this resolution, however, it looks like overkill to me, as it would be a 320+dpi res screen. I believe most printing is done in 300 dpi resolution, so this just sounds crazy to me.
Furthermore, if we consider just for a moment that they would use something like 720:480, and people start developing apps for this screen res, all the iPods and iPhones up till the 3rd generation will be left behind. I agree, that at some point they will probably have to do so. But what about the iPad? With this kind of resolution, the iPad x2 function is useless. Hence those apps would look really weird on the iPad screen, with a nice big black bezel. There would still be a bezel with the 960:640 res, but i guess it would be better.
Costs of the new device?
With all the new features, better cam, better proc, front cam, alu design etc. the costs of the 4G will be higher, so in order to be able to sell the phone at the same price as before, with the same margins as when the 3GS was introduced, I just can't see them putting in a really high res screen this year. My guess is this could be an update option for next year.
The 2x feature in the iPad os
I don't think that they developed this feature for iPad only, but that they had a change in the screen res of the iPhone in mind, so that they would be able to smoothly change the res, without making the existing iPhone/iPod Touch apps look weird.
Bottom line I think that if we see a screen res change, it will be to a resolution of 960:480, but I don't think that we will see it this year, more likely next year.
Any thoughts on that ?