Modified. Sorry for those of you who saw the previous versions.
ETA: this is the current state of things, not exactly what will eventually be implemented. As we all know ad implementation is a WIP :)
pmmarcov also indicates that "People want to change the colors of the Google text ads too and [they]'re working on ways [they] can give that choice as well."
Plus users having s2 layouts will be able to choose whether they want to have vertical ads on one side or horizontal ads at the top.
Will one of the horizontal ads remain whatever you've selected?
Yes. People selecting horizontal will get ads at the top and at the bottom whereas people selecting vertical will get ads on one side and at the bottom. So the bottom ad is for everybody. Logical.
Which ads?
Vertical is the existing Skyscraper (160*600).
Bottom ad is the newly created Graphic Leaderboard (728*60) for people having selected horizontal.
Bottom ad is the existing 5LinkUnit (728*15) for people having selected vertical.
Sources:
http://community.livejournal.com/changelog/3373865.htmlhttp://community.livejournal.com/changelog/3384702.htmlhttp://community.livejournal.com/changelog/3384371.html