When you want to text someone that owns an iPhone with iOS 5 with your iOS 5 iPhone, it will always try to send it as an iMessage. It will use your data plan or your wi-fi, saving you some money.
That's good, but sometimes you already know that the recipient is out of data coverage and you don't want to wait the iMessage timeout before sending it as a normal SMS.
The trick is: send your message as usual, then tap and hold the message and choose Send as Text Message!
You can do it since the delivered label will be displayed under the message!