Another learn from my mistakes post:
Recipe for creating chalkboard paint here at Martha Stewart's.
And I guess Martha really knows her stuff, because she was right about using a roller or sponge (and not hair) paintbrush. I didn't have a sponge paintbrush around, so I used a regular brush. Mistake.
Regular brushes leave a lined texture (see picture) that is not fun to write on (real bummer: what good is a chalkboard if it isn't fun to write on? :-().
(It also might be wiser to use a darker shade of paint.)
tip: I found acrylic paint for the project to be expensive since it comes in large containers. My solution: buy a tester container. It turns out to be exactly enough for a big board.