I found this in the Winter 2007 "homebasics" magazine that came in the mail. It can also be found online (when I couldn't find the magazine the other day). I decided to post it here for future reference.
1 teaspoon butter = 3/4 teaspoon olive oil
1 Tablespoon butter = 1 1/4 teaspoons olive oil
2 Tablespoons butter = 1 1/2 Tablespoons olive oil
1/4 cup butter = 3 Tablespoons olive oil
1/2 cup butter = 1/4 cup + 2 Tablespoons olive oil