We once hosted a big mexican themed fish bbq party and a friend who owns a mexican restaurant was hooking us up with a bunch of food and a margarita machine. I showed up to pick up the food and noticed that they had forgotten to prepare their famous pink sauce. The owner called over to a cook and told him to make a batch of pink sauce asap, so I got to watch him do it. Here's how they did it:
1 quart fresh salsa
1 quart Best Foods Real Mayonnaise
Mix together with a big spoon until you achieve an even consistency.
Don't tell anyone that the sauce is 50% mayo.
It's actually pretty amazing. I personally wouldn't try adding sour cream, since that adds a pretty distinct flavor.