As was mentioned a couple of days ago, it was though Blizzard would be doing something special at the Rudolfplatz in Cologne. While some fans have been (hoping?) speculating that it might be a beta announcement, it seemed pretty clear to us it would just be some promotional thing.
News today on German site Justgaming (thanks to Thorsten for the tip) says it’s going to be Diablo/Blizzard inspired street art of some sort. I’ll try and head over there and take a peek if I get a chance and if you are going to Gamescom it may be fun to check out.