I did play with AA the whole time, today is the first time I encountered this issue. It does not appear when AA is turned off.
http://imageshack.us/f/18/falloutnv20110619234540.png
http://img232.imageshack.us/img232/2495/fa...10619234603.png
And no, I'll not play without AA since a) I've played just fine with AA until now, and B) I can't stand stairs.
