http://french.about.com/library/writing/bl-texting.htm

As you can see here, “RE” comes from “de retour” – I’m back.

Edit:

As Laure commented, we can add “re-bonjour” which means “hello again” and is similar to “I’m back”.

You would use “re” also in person on an informal level when meeting someone you already have greeted on that day (“Bonjour !”) and want to repeat… (“Re-bonjour“). The chat use seems parallel to the common practice.

You may turn re as desired, personnaly I consider that re = ^^ reloaded (Yes I am a geek: D)