If your phone's screen somehow looks like it has a checkerboard pattern, with serrated text and geometry, and dithered reds, chances are you're looking at an OLED screen with a PenTile matrix.

What's that? Well, insted of the normal matrix, you get a special kind of matrix where one pixel relies on adjacent subpixels that are not its won to reproduce colours. This saves OLED manufacturing costs and longevity, but it does come at a cost to overall image quality, especially text.

If a normal matrix is like this for a 3x3 grid

RGB RGB RGB
RGB RGB RGB
RGB RGB RGB

Then a PenTile matrix is like this:

RG BG RG
BG RG BG
RG BG RG

You're looking at bigger R and B subpixels.

Personally, I'm very sensitive to it. Even at 306 PPI, PenTile is too obvious.

So how exactly would white work? Would it have to dither the green as well?

Reminds me of some of the cheap CRTs that shared beams and often looked blurry.

My Galaxy Nexus is one of the devices that is listed as using PenTile, so I decided to take a very close look at my display. Sure enough, I could see the checkerboard pattern, but it's so small that you have to have very sharp eyes to really notice it.

For comparison though, here's this:

The left half is the Galaxy Nexus's PenTile screen, the right the HTC One X's Super IPS LCD2 screen.

I also found this little tidbit for you Twigz:

Yes, the original Galaxy S has pentile. There are two different types of pentile, RGBG and RGBW, its RGBW that looks terrible. That type is not used on AMOLED displays. I have the HTC Incredible and the earlier versions used the original AMOLED and it looks great to my eyes too. Compared to my friends Bionic (with RGBW), you would never know it uses pentile.

Bionic's PenTile RGBW article

Basically it's a 2x2 square of pixels with Red, Green, Blue and White.