Tomorrow Sony is expected to unveil a new Sony PS3 Slim console along with PS3 price-cuts. At the Gamescom 2009 Sony has actually blocked 3 hours for their press-conference in Cologne according to the Gamescom site (starts 6pm local time).
3 hours? There must be a lot more than just a new PS3 Slim and some price cuts. New games and possibly new Playstation Network services could be part of the announcements.
As the Sony Gamescom 2009 conference is at the E-Werk in Cologne some music and show elements could be part of this press event. The E-Werk is an event location that hosts lots of concerts.

Sony needs some big announcements for the PS3 to get back into the game this Holiday shopping season.
Nintendo is apparently not announcing anything at the Gamescom 2009 as they have no press conference scheduled. So it is just between Sony and Microsoft (Wednesday 10am).