I have seeded over the top of old grass without a slit seeder. No soil preparation whatever. It did take fairly well, but best "take" was when I used triple the usual pounds of seed per thousand. This is difficult to determine how well the "take" has succeeded, because except in the first two weeks, you usually cannot tell the old grass from the new grass.
In my test I seeded 2 sq feet at triple and 2 sqft at the usual rate. October 11, 2011. Seed was Scotts "Classic".