The bug is replicated by sending the following string of text to another user via iMessage:
لُلُصّبُلُلصّبُررً ॣ ॣh ॣ ॣ 冗
A lot of people on twitter seem to suspect that the Arabic text is causing the phone to reset. This, however, isn't the whole story.
How it works.
Springboard realises that it needs to crash and restart to protect and fix itself (In the same way your computer will show the Blue Screen of Death, kernal panic, or your web browser will close the webpage you're viewing telling you that something went wrong).
Secondly, this phone will only restart if the person receiving the message isn't already in the Message app. This is because it's the notification that is handled by springboard and causes it to reset.
How to Fix my iPhone.
You need to get those unicode characters removed from the preview pane of the message app, so that your phone doesn't try to read them when you open the app and crash again. There are a couple of ways of doing this.
1) if you have Siri enabled, dictate a text message to the person who sent you the message. "Hey Siri, send a reply to my latest text saying 'thanks, your tried to break my phone. Aren't you a cool person'. " should do the trick.
2) You can go to any app that allows you to share something via text or iMessage (such as the photo app) and send a photo to the person who sent you the message.
Again, the aim here is to remove the string of characters from the message preview pane.
3) I'm gonna go out on a limb and say that the Mac Message app isn't affected by this, so you could potentially open the Message app on your Mac and delete the message there and wait for it to sync across to your phone.