Interior paints have a scrubbability rating, established through standardized testing. This is a good indication of a paint film's toughness and ability to withstand physical abuse. Though this rating may not be posted on the can, a paint retailer should have information on the rating. By comparing these, you can get a good idea of the paint's quality.
One problem with using a flat paint on interior walls is that it can be washed, but it doesn't take kindly to scrubbing. If you scrub it with a damp cloth, you'll remove the dirt or smudge but exposed pigment particles actually become burnished or polished--which ruins the finish. To avoid this, it's better to choose a high-performance eggshell (not flat) paint.
Some new high-performance finishes are amazingly easy to clean--you just sponge them off as if you were wiping off a countertop. Ketchup, food, scuff marks, mud...all of these things just wipe clean. Sherwin Williams' Everclean is such a finish; Dutch Boy Kid's Room Paint and Benjamin Moore & Company's Regal AquaVelvet and Sears Best Easy Living Satin are other easily-cleaned examples.
--Don Vandervort