Many have asked the question and there's really never been a great way to deal with it...creating a wall that follows the slope of a spiral or curved stair. Well, with the introduction of the Swept Blend in Revit Arch 2009, it can be done quite easily!
Once the program is officially released, I will post more detailed instructions on how you can create something very similar to what I have below... I really just wanted to give you a taste of what's to come with RAC 2009! Since the release of the product isn't finalized (as far as I know), I don't want to spell out the details just in-case something changes with the feature.
Contents of the Swept Blend Sketch... (One of the keys is having the profile high enough to cut the entire wall's height)
The Finished Swept Blend...
The Finished Product! (Yes, I modified the door to show open for this image :))