At the moment, I'm passionate about rock climbing, travel, surfing and computer programming. If it weren't for that 4th one, I'd be screwed.
I currently run maybe the 3rd most popular Travel Blogging site out there. It's had a good 5 years of passion poured in now, off and on, and if I were to slap ads on it today I'd pull in a buck thirtyfive a day.
I also run a little service that processes the draconian logfiles that Amazon provides for its webservices and spits out pretty analytics. It's a subject that no human being could ever be passionate about and it pays my rent.
Sometimes it's better to monetize something boring that people are willing to pay for.