Yes they are, and I've seen my wife (biology professor) demonstrate this at a city event in front of TV! There are a few folk recipes that use acorns, but yes they are very bitter due to the tannins they contain.
Quericus (oaks) are a large genus of trees. I would not like to rely on ALL of them being edible. Seek out a local expert on your local oak species first...
My wife adds: she thinks all of them technically are, but whites are more palatable than red or black acorns due to the tannins. White oaks have smooth lobed leaves, red oaks have points on their margins.