Thought I'd just add to this a bit with some numbers since percentages don't always get picked up well.
January 1, 2008:(21 weeks) 280.6% increase (near quadruple) About 300k more a week than the 360
April 1, 2008: (34 weeks) 176.1% increase (near triple) About 180k more a week than the 360
July 1, 2008: (47 weeks) 129.4% increase About 130k more a week than the 360
October 1, 2008: (60 weeks) 102.9% increase (more than double)Â About 100k more a week than the 360
January 1, 2009: (73 weeks) 85.9% increase About 85k more a week than the 360
April 1, 2009: (86 weeks) 74.0% increase About 70k more a week than the 360
July 1, 2009: (98 weeks) 65.8% increase About 63k more a week than the 360
October 1, 2009: (112 weeks) 58.5% increase About 55k more a week than the 360
January 1, 2010: (124 weeks) 53.5% increase About 50k more a week than the 360
Add in for comparison that last week before the Xbox360 semi annouced price drop and with the PS3 just 4 weeks into its 100 dollar price drop the 360 was outsold by about 40k and during the largest non launch week essentially which was last week for the PS3 it outsold the Xbox360 w/w by just 57k its hard to justify a huge increase in sales. Also of note if you look back at a fairly huge game for the PS2 like FFX you can see that it increases sales for about 2 weeks and no where near 100% of double sales it increased sales prior to its release by 77% I'd imagine that is what we would also see from FFXIII or a game like Metal Gear Solid. Anyways make your own judgement from that information and the fact that it took the Wii at its current levels of sales what will be more than 9 months to catch the 360.