I heard about this at a WW meeting and wanted to try it out. It's a yellow cake mix and instead of adding the eggs, oil, etc. add a can of diet Sprite (or the Walmart version--Twist Up) and bake as normal. It sounds awful but I tried it last night and it turned out to be really good. And the best part it's only 4 points for a reasonable piece. I frosted it with fat free cool whip.
And I hear that if you want to make a chocolate cake to use a can of diet Dr. Pepper.
I may be the last person that's heard of this but I wanted to share it, just in case you are looking for a way to have your cake and eat it too. :)