From Ask Cecil:

The rear wall ... is parabolic in cross section when viewed from above, and the porcelain finish is conducive to laminar flow. The principles of fluid dynamics tell us that a fluid striking a smooth surface at an oblique angle will tend to flow along that surface. Assuming the source of the fluid is near the focal point of the parabola... the fluid will run straight down the wall with little or no splashing.

He is, of course, discussing a urinal, in response to a question about where to aim. I'm glad to hear that considerable Science goes into the design of these. I only wish equivalent Science went into making sure that toilets flushed reliably, but judging by the homes I've lived in, that isn't yet entirely successful.