http://docs.info.apple.com/article.html?artnum=22194
...Therefore, a certain number of subpixel anomalies is considered acceptable. Rejecting all but perfect LCD panels would significantly increase the retail price for products using LCD displays. These factors apply to all manufacturers using LCD technology--not just Apple products.
If you suspect your display contains a high number of pixel anomalies, take your Apple product to an Apple Authorized Service Provider for closer examination. There may be a charge for the evaluation.