I don't think it's a bad idea, take a look at the history of Massive (http://www.massiveincorporated.com/ http://www.massiveincorporated.com/) before you start

I don't think it's a bad idea, take a look at the history of Massive (http://www.massiveincorporated.com/ http://www.massiveincorporated.com/) before you start out. From what I understand they attempted to put big brand ads into the maps of AAA games and haven't been very successful. I know this isn't exactly what you were thinking, but there may be lessons to learn from them.