I figured it out an important feature to save you money!!! The automatic setting on iMessage is that when iMessage is NOT available (Server is Busy), it automatically sends as an SMS text instead, thus charging you money! You can turn OFF this feature. iMessage uses a internet as a server to send the text as you can imagine that the server can get busy with everyone using iMessage.
You have to goto Settings- Message - iMessage - turn OFF "send as SMS text if iMessage is not available." Take a look. Once the feature is turned off the iMessage server will keep the text and send it when the server is not busy. There may be a very slight delay, but you save money.
Clip from article:
"SMS texts you've sent will show up with a green background, while iMessage texts show up in blue. While you'll still have to pay the traditional fees for any SMS texts you send, it bears repeating that all iMessage texts go out over Wi-Fi or 3G and are completely free."
http://www.tuaw.com/2011/10/12/ios-5-features-imessage-brings-free-texting-to-ip ad-ipod-touch/
(Note- Data usage charges apply)