One of the coolest new features of iOS 5 is iMessage. iMessage is a messaging service by Apple that allows owners of iOS 5 to send free messages between their iOS 5 devices. These messages can include images and videos and are much better quality when sent over iMessage than over the phone carrier's MMS service. Not only that but iMessage gives a user a status of the messages when they are sent if they were deliver yet by displaying the word, "Delivered" below the last iMessage that has been delivered.
This is very nice but another great feature that helps is the Read Receipts! This is a feature that is turned off by default but basically lets the sender know when/if the recipient has read the message. It does so, not by notification center or any type of push notification, but by marking the last read iMessage as "Read" instead of "Delivered".
For others to see when you've read their iMessage, go to the Settings app, select Messages, and then turn on Send Read Receipts. To see when someone else has read your iMessage that you sent to them, you need to get them to turn on the Send Read Receipts on their device.
This helps in the situation where when someone sends a message to another person, they can know when the message has been read instead of asking, "Did you get my message?". I encourage everyone to turn this on as it's a convenient and courteous feature.