How will I have to update how I program for iPhones now with iOS4?

For example, preparing a launch screen of 320 x 480 would have to be changed....

How is that going to work for us? Are programmers always going to have to be submitting a high开发者_运维知识库-res that will be scaled down for old devices such as the iphone 3g?

The size of the screen is basically 4 times on a pixel by pixel basis. So each pixel of your image for example gets boosted to 4.

What this means for you? You don't have to change your App, your app will scale to the hi res screen for you, same with your UI and images within your UI. Of course if you want to take advantage of the better screen quality you will have to submit hi res images.

I haven't looked at going the other way but I believe it would be a similar case.

One exception to this is text. It automatically scales to the higher res for you for free. So text will look super sharp. One problem with this is if your loading image has text based on the original load screen that wouldn't look the same as when the high res text loads.

Strictly speaking, anyone who's seen the documentation on how they're handling this is still under non-disclosure until Monday, when the new iOS ships.

Suffice to say, it's clever. You'll be able to put both high and low-rez versions of ALL your images into your app, and then load them into your app in a way that's totally transparent from the code side. The device will make its own call about which version of the image is appropriate for the kind of screen it's got.

Now that the WWDC 2010 videos are available for free to any registered iPhone developer (or ADC member), I recommend watching Session 134: Optimize your iPhone App for the Retina Display for a full description of what you need to do to support the iPhone 4's new display.