I've also been searching for a Mexican white cheese dip. I was able to find this one online, but have not yet tried it.

CHILE CON QUESO
Yield: 1 1/4 cups

1 cup asadero cheese; shredded*
4 oz jalapenos, chopped; drained, or 4 oz poblanos, deveined & julienned
1/4 cup half & half
2 tbsp onion; finely chopped
2 tsp cumin, ground
1/2 tsp salt

Heat all ingredients over low heat, stirring constantly,
until the cheese is melted. Serve warm with tostaditas.

(If this is made with julienned poblanos, it's called
"queso con rajas de chile").

More information on this recipe and the background surrounding Mexican white cheese dips can be found at: http://www.xenite.org/white_cheese/