Cooks Illustrated/America's Test Kitchen had a hummus recipe a few months ago (looked it up: May - June '08 issue). The trick to smoothness is to make an emulsion like mayonnaise. When you're doing the food processor bit, chop up the solid ingredients first, then add the non-oil liquids (lemon juice, chickpea broth, etc.) and finally, in a slow stream, the opils (olive oil, tahini).

Topping hummus with sumac is also good.