Considering the Playstation 2 hit the 1b s/w mark....It's probable the X360 or PS3 could hit 1b. The Wii has the best chance *if* it's tier ratios started improving (but they're not).
Its very unlikely the DS would hit the 1b mark. The DS has had a lock on a 5.0 tier ratio, and has stayed there. The DS would have to hit 200m units sold to attain 1b s/w sold.
Either way, we're either going to have to see a system have a global tier ratio of 14.0+ for the PS3/360, or the Wii is really going to have to do something like either sell 100m+ systems, or have a 360-like tier ratio.
Right now, the 360 is way out ahead of the pack, for console software. In the US, it's sold around 40m units. Add on another 25m from Europe and Japan, and it's at 65m. The Wii is *around* 30~35m, and the PS3 should be around 10m by now.
Back from the dead, I'm afraid.
