In a skillet cook the bacon over moderate heat, until crisp. Transfer bacon to a plate lined with paper towel and pour off all but 2 tablespoons fat. Add onion and garlic to skillet and cook, stirring occasionally, for 5 minutes. Add cumin and chili powder and cook, stirring, 1 minute. Add beans and cook, stirring occasionally, 5 minutes.
Transfer the mixture to a food processor and puree until smooth. Add the cheddar, sour cream, lemon juice, salt and cayenne and puree until combined well.
Transfer dip to a serving bowl and garnish with the bacon, crumbled.