I saw this in the paper from Mary Hunt's column entitled: Ordinary Ingredients Can Produce Magic Results .
If you haven't experienced the power of original blue Dawn plus white vinegar, you are in for a treat. Remove the top of a plastic spray bottle, and fill half of it with white vinegar. Carefully heat it in the microwave until the vinegar is hot, taking care not to melt the bottle.
Fill the rest of the bottle with blue Dawn liquid dishwashing soap. Apply the top tightly. Gently shake the bottle to mix. That's it. You now have a powerful cleaning product that will melt soap scum and tub and shower buildup and clean sinks, appliances, and just about anything else. Just spray it on, scrub, rinse and be amazed. For tough soap scum buildup, spray the mixture on and allow it to sit as long as overnight. The scrub and rinse.
9/17/10 I made this and used it to clean my tub and stovetop. I thought that 50/50 ratio of Dawn to white vinegar is too strong on the Dawn. I since have tried a 5% Dawn to 95% vinegar ratio and it worked a lot better. The other way was just too soapy.