Apple's internal policy on acceptable numbers of pixel anomalies was recently updated. The policy is used as a guideline for Mac Geniuses addressing customer issues that involve Apple displays. It essentially allows them to quickly and easily determine whether or not to repair or replace products with display defects.
Apple provides its employees with a table showing the acceptable number of pixel anomalies for each product range. The table also outlines the allowable numbers of pixel anomalies for Apple's MacBook, MacBook Pro, iMac and Cinema Display lines.
Of note, Apple's policy allows Mac Geniuses to offer exchange services even if pixel anomalies are within acceptable ranges. If the customer receives a replacement product with even more anomalies, but it's still within the acceptable range, further exchanges aren't permitted.