OT: are 72dpi still state of the art for screen design?

[Colin Holgate]

If you are drawing text, and want the text to occupy an exact amount of screen area (perhaps because you are doing a page layout, and want to hold up the physical paper against your screen and see that it's the right size), then dpi may matter. If you're working on graphics in Photoshop, that are to be printed to a 300 dpi printer, then you may need that dpi in the image, and it would have nothing to do with the size of your screen. In the case of a bitmap that has an exact number of pixels width and height, and could be going to any sort of device, you would want to work in a standard way, and an easy way to do that would be to work in points per inch. A 'point' by definition is 1/72 of an inch. Hence seeing 72 pixels per inch being used.

So, don't worry so much about dpi of images, just have them be wider or taller. For example, if you want the same background image to fill the width of an iPad, iPhone 4, and iPhone, it might be 1024 pixels across, 72 dpi, but by the time it appears on iPad it will 132 pixels per inch, on iPhone 4 it would be 326 pixels per inch (but actually the image is scaled down to 960 across too), and a 480 pixel image at 163 pixels per inch on iPhone.