Here's what I think is going to happen from the "Big 3" press conferences (in no particular order)

Microsoft

• Microsoft will show gameplay and date Halo 3: ODST
• Announce that the upcoming Grand Theft Auto IV: The Ballad of Gay Tony will be "exclusive" DLC
• Gears of War 2 receives next significant DLC.
• Unveil the much speculated Motion Capture Camera and that it will be packaged with a new fitness "game".
  
• Modern Warfare 2 will be an exclusive to Microsoft for a week before being released to all other platforms.

• Sony announces PS3 price drop
• PSP2 (or PSP Go) will be announced featuring touch screen, no physical media, and battery life longer than DS.
  
• A significant LittleBigPlanet expansion will be shown and vaguely dated for holiday 09' (Significant as in several new single player levels, not costumes)
• God Of War 3 will be demonstrated on stage.
• Metal Gear Solid 5 will be announced. Protagonist will be Grey Fox.

• Reveals a new Legend of Zelda trailer that will utilize Wii Motion Plus and Wii Balance board
• Wii Fit 2 will be announce and (unfortunately) demonstrated on stage
• New Super Mario game announced with a trailer.
• Tony Hawk Ride will be balance board compatible, with simplistic controls included.
  
• 1080 snowboarding for the Wii will be announced