As I've thought about this more, I don't see any reason we'd want the viewport to be any different from other scrollable containers WRT sticky behavior. As long as the containing block is not clipped, the sticky element shouldn't be either.
Created attachment 170024 [details] simple test case
Yes, we plan to do this.
<rdar://problem/12571189>
Created attachment 202249 [details] Patch (not ready for review)
Created attachment 202333 [details] Patch
http://trac.webkit.org/changeset/150395