Probably never.

 Not neccaserily , a better attach rate can help the PS3 outsell the 360 although the PS3 userbase would still have to be within 10-20% of the 360's.

The easiest answer: if and when HW sales will surpass XB360, almost surely weekly SW sales will surpass too, but given the advantage gained, total SW sales will surpass after a while, and if the weekly advantage won't be enough, PS3 will need to sell some time after XB360 ceases sales to make it happen.
Two thing that could help to accelerate times would be saturation and, during its last years, fading of XB360 and PS3 WW sales getting so good to increase the release of good titles.

@Shameless: I partially agree, if PS3 doesn't manage to have an higher weekly advantage, HW and SW total sales will exceed XB360's ones only if Sony keeps on selling PS3 for a while after XB360 is phased out, so it's quite possible that it won't happen during XB360's life, but "never" is excessive.

Gears 2 will be bigger than MGS2.

Fable 2 will be bigger than R2.

Too Human is on par with LBP.

What is M:PR? I don't know but I'm sure that Banjo 3 will be bigger.

Geez, you both have certitudes    

I dunno, with week to week software WW of the 360 outselling the PS3 only 1.23 to 1, even though the 360 has about 1.5X the number PS3 consoles sold WW. I think that we will soon see the PS3 surpass the 360 WW with week to week software.

For NA, the 360 has sold 1.63 to 1 last week.....this will take longer, Perhaps until the end of the year we will see this equalize.

lol "clutching at straws"

 /Agree, the huge gap that seprates the PS3 and 360 is almost all from the U.S and the gap is closing; but only due to WW sales. The PS3 most likely can beat the 360 WW if it manages to maintain its current rate. 

This isn't 2006 anymore......the 360 is going down, and hard

Its quite possible that the Xbox360 will continue to outsell the PS3 in the NA market for at least another 6 months. I would say that it will never be outsold on software by the PS3 in NA.