Adding my two cents onto this: The first stairway I successfully made was one carved out of the floor onto level -1. To complete the stairway, instead of carving an up stairway out of the remaining rock, I carved an up/down one. Why I didn't carve an up/down one on the main level I don't know, guess I was still getting used to it.
NOW I want to extend the stairway up to level +1. Although I've canceled and restarted it several times, the best I can do is have my mason walk up, get the construction to "nearly complete", and then suddenly suspend construction because an item is "in the way". I try it again two tiles over to the right, works perfectly. I suspect the offending item may be the piece of flint that's been sitting on the staircase since I first carved it out, that everybody refuses to touch. I set up a garbage zone for (D)ump-ordered rocks to go, and while every other one has been taken there, that one remains in place, hovering on the stairs. I'm positive it's not set to Forbid, so I don't know what its deal is.
Edit: While I was at it, I deconstructed and reconstructed some of the other parts of the stairway to see if that would help, no go -- but now I notice the formerly up/down stairway is just displaying as an up stairway, even though it still retains the properties of an up/down one. Obvious suggestion that detection of this be added in future to change stair icons to the appropriate X where applicable.
Edit again: Now that I think of it, I also got an item-in-the-way error out of the blue when trying to build my mechanic's workshop, except when I unsuspended that it continued as normal. I was trying to build it on some scattered flint rocks too.
[ October 31, 2007: Message edited by: Munkee ]