I read your post at Rotten Wood Stairs (for others to refer to as well.) I did see some replies there too.
Did you take the suggestion of asking that the stringers be replaced, to the Builder? If so, what did he say? Also, if you want, you can ctc. your Municipal Housing Inspector to stop in. Tell him you think they're 'unsafe', and that he ought to refuse a Certificate of Occupancy (when the time comes.) If it comes to the latter, I wouldn't tell the Builder, but somehow, the Inspector will have to 'find' the damage. In order for him to do so, your stairs CANNOT be finished/carpeted/whatever. He'll need to see what you see.
I don't see why you can't go to the Inspector now to see what your options are. Sure, wait for others to reply, but time is important.
My best to ya and hope this helps.
Jay J -Moderator
PS: Of course, we'll ALL be interested in hearing how your problem was finally resolved. Be SURE to post back w/the 'answer' ...