I have to say, I've fondled a few new phones lately that were just really exciting and the sole reason is because of the bright new QVGA screens they all sported. (QVGA stands for Quarter VGA, or 240x320 resolution, in case you're not hip with the lingo). I'm telling you, it makes a *huge* difference. Now that I can see the pipeline of handsets filling up with QVGA screens, I can really say without a doubt that they are going to make fundamental improvements in mobile usability and thus higher usage. It seems like there wouldn't be much difference between, say a standard 176x220 screen that many phones have today, but it's night and day. Not just because of better eye-candy, though there's lots of pros in just that alone, but because there's just so much more information you can fit on the screen, so much more subtlety you can add to graphics and widgets, and generally just a total increase in the instant satisfaction you feel when you're using the handset. People see a QVGA screen they just say "wow".
Want an example? The new iPod with video. That's a QVGA screen, and it's caused a complete industry wide landrush into portable video as a result. But the brighter, nicer screens I've seen on handsets just make the iPod look dull by comparision. Just brilliant. Now, physically, the QVGA screen could be any size. The QVGA-sized example squares above could be tiny or huge depending on your monitor... there's no way to really show you exactly the size they'd be on the phone (that I know of). Generally, the phones I've seen lately have pretty big screens (fat pixels), though I've seen others that were the same resolution but tiny as today's screens. I don't think that size is as good, but it does have the effect of enhancing the resolution (tiny pixels) and really makes the screens pop. To me though, screens that your Mom can read without her glasses is a good thing. The screens still aren't the size of Compaq PDAs or anything - just reasonable sized mobile canvases on which to draw your information.
Play around with it yourself to see. Create a web page with a div that has hard-coded height and width, and then make the font 8 or 10 point (about the size of a phone's font) and see how much text you can put inside that rectangle. It's a surprising amount of info, actually. And there's enough room to add in navigation and GUI elements to a page, without making it feel cramped as well.
And again, the iPod has shown that there's an appetite for mobile video in that resolution too - the guys at Modeo are geniuses for using that same screen size and resolution (down to the millimeter) as the basis for their mobile video phone. And like I said when I first talked about that device, I think the wide screen format might be great for browsing the mobile web as well. We'll have to wait and see (as it also depends on the ability of the handset's web browser), but it seems like a real no-brainer to me. No matter what, I think QVGA could have as big an impact on the mobile market as color screens and cameras did just a couple years ago, and that's pretty great.