Perhaps it's a contrarian thing?

Most developers use whiteboards, so the exceedingly tasteful folks at 37signals will surely use something more refined.

I don't like the look or sound of whiteboards. They don't age well either. I love the feel, sound, look, texture, and patina of chalk on a nice dark chalkboard.

I love chalkboards. It's a tactile thing. They're more work to keep clean and slower to clean, but I keep a blackboard at home instead of a whiteboard.