If PS3 can out sell 360 worldwide by 100 000 units a week, then by 2011 PS3 will have the larger install base.
Now if PS3 can out sell 360 worldwide by 75 000 units a week, then by E3 2011 PS3 will have the larger install base.
Its hard to predict base sales, but by 2012 PS3 will for sure have a larger install base.