24-bit color means 16 million colors can be represented (2^24 = 16,777,216). Some displays are limited to 18bit, 16bit, etc., which limits the number of true colors, meaning some colors are resolved by dithering (blending pixels to create unrepresentable colors).
I thought the non-S, AMOLED was 16-bit. I'm 99% sure the iPhone display is 24-bit too.
[edit]
Just saw the post about Atrix "firsts". I think regarding the 24-bit, it's a first when combined with the qHD resolution, so I'm thinking there's probably some other 24-bit display Android devices, just not at 960x540 (which is qHD, Quarter HD, 1/2 of 1920 x 1/2 of 1080).