Typically when you send an iMessage from your iPhone to another, if it doesn't get to the recipient in a timely manner, it will automatically be sent as a text message, but sometimes that fallback just doesn't work. Thankfully, there's a way to force your iPhone to comply with your desire to send a message with SMS.
Let's say that you send an iMessage from your iPhone to another iPhone. You wait a few moments only to find that the message never gets marked “Delivered.” In my experience, the iPhone doesn't always automatically send the message as an SMS. That, or sometimes it just takes a while.
To force iMessage to send via SMS, tap and hold the message bubble, select “Send as Text Message,” and marvel at the immediately sent message. If you don't see the option straight away, try tapping the arrow on the right of the popup bubble to cycle through the options.
“Send as Text Message” in Messages on an iPhone
The primary requirement for this to work is that you have the contact's phone number listed in their info. If you don't, your iPhone won't offer this option. On a related note, you can also select “Send to Email Address” if the contact card has an iMessage-registered email address attached to it.
The next time you're frustrated with iMessage taking it's sweet, sweet time to send a message, you now know what to do.