I think the potential for Sony to outsell Microsoft is great especially over the next three years. Sure right now MS has the Slim effect which has boosted sales, they also have Gears 3, Fable 3 and Reach coming, and finally Kinect that is a big 2010 and early 2011. Then what do they have? Pretty much nothing, with no new Halo, Gears or Fable they have no new system seller.
Meanwhile the Playstation has a very strong lineup to match with Move, Little Big Planet 2, Killzone 3 and Gran Turismo 5. Those titles should help them weather the 360 storm this year. Then you got the Agency, Infamous 2, Uncharted 3, Resistance 3, Last Guardian, Twisted Metal, FF VS. XIII, and FFXIV which should help them. They also have Blu-ray which is getting better attach rates all the time, 3D, and the DVR kit not released in America.
Kinect and 3D are the big wild cards, if Kinect is a system seller Sony faces an uphill battle but if 3D catches on at home like it has in the movies Sony is in a great spot.
I believe that because Sony has much better first party support, Japan locked up and a very strong presence in Europe and the Middle East (UK exlcuded) that they will be able to surpass the 360 Worldwide.