A year ago, Apple up the ante with the Retina display. Close to a year later, I see nothing from its competitors that come close. We might just see one from HTC's Pyramid, or Sensation on T-Mobile coming this summer, but it still will not be matching Apple's DPI. By now, I would have expected 720p displays on those roomy 4 to 4.3 inch displays we regularly see on Android devices.
So, where 720p resolutions? It seems that non-Apple devices have topped out at 800 x 480 (from 3.7 to 4.3") with the HTC Sensation coming in at 960 x 540 on a 4.3". The iPhone has a tighter range with a higher resolution at 960 x 640 with a 3.5 " screen. This still give the iPhone 4 a better screen, hence, the label Retina display because you cannot see the individual pixels like you can on other devices.
Again, by now, I have expected Apple's competitors, especially Samsung, to outshine the iPhone. For mobile devices with a screen size between 4" to 4.5", I think the 720p would be a nice fit.
You can bet that if Apple ever releases an iPhone with a bigger screen as some hopes and probably will, they will not stay at the same resolution as the current iPhone 4. It's time for HTC, Motorola, and Samsung to step it up with their display resolutions. It's 2011 after all.