AFAIK, the beagleboard supports 24 bit colour, and uses omap3, so the n900 probably does too. OpenGL ES (3D acceleration) only works at 16bit colour depth though, and this is probably the reason why this is the colour depth used. Plus the screen will only be 18bit depth at the most anyway like all TN TFT panels. I somehow doubt they put an IPS or PVA panel in a phone...