PS3 should certainly see an increase as it's first half sales in '10 will definitely beat the first half of '09. GT, GoWIII and FFXIII should all help boost sales too. Second half will probably be lower than '09 but up over all YoY.
I expect a decline in 360 sales at least until a price cut and/or Natal. Then it could see a significant boost. Too early to say how much NATAL will change the game though.
Wii is hard to say. It's back up at the moment. NSMBW will continue to sell really well. Nintendo has SMG2, M:oM and Zelda Wii all in '10 but I don't see any of those pushing hardware sales as that market already has Wiis but any hold-out GC fans will definitely be aboard at least. Vitality Sensor games may increase hardware sales if they create new buzz. Again too early to tell. Black Wiis in NA could help as well. A decline is certainly possible, even probably but I think 20% is too much.