Well, completely flat and round at the same time.

I've used a radius block on a couple of fingerboards after removing the frets and you'd be surprised at how poorly radiused some are.

If you do want fret markers on a fretless neck, then it can often be easier (given relatively basic skill and tool levels) to get a fretted neck and fill in the slots with a contrasting coloured hard filler e.g. coloured epoxy, to give you the markers, hence Rabbit's comments.