Hi,
I sanded mine to 320. If you want it really smooth, get 600 grit wet or dry paper (black color, in the automotive dept) and wet sand the board with your preferred finish oil (after dry sanding through 320). This gets the wood sinfully smooth and silky. Keep the sandpaper fresh and oil-soaked as you work.
Casey

I would not bother going finer than 120 or 150. The surface is going to get scarred on first use, assuming this is going to be a "working board."

Thanks, casey and bobs, for your responses. I think I will stop at 150 grit and then apply mineral oil. Yes, it will be a working board, so it'll get sliced and diced daily. Today I looked at new cutting boards being sold at Williams-Sonoma, and they were quite rough--certainly no smoother than mine is now. Their display tables, on the other hand, had sealed butcher block tops that had been sanded, as Casey describes, to a silky sheen.

Thanks again!