Blizzard has trademarked "Cataclysm" a few days ago. You can find the official filling for the patent at the United States Patent Office's website here.

What does that mean to us? Well many amongst the community feel this is a sign of the next expansion which might be announced at the upcoming Blizzcon 2009. There are a few signs of proof for this, according to the wiki page for the Great Sundering at WoWWiki states the event that caused the Maelstrom has also been called "Cataclysm".

Here's to hoping!