I saw your request and I remember seeing a cake like this before - even though I've never made it. Go to www.allrecipes.com and type in "pop" in the search section and you get 4 different recipes like you requested.
That recipee is always floating around on the newbie board on the weight watchers website. I have never tried it- but I have seen it many times while lurking....
Use diet cola, any kind with Choc. cake and use diet Mt Dew or simular soda with a yellow cake mix, spice cake mix etc. I guess if you had a strawberry cake mix you could use diet strawberry soda.