It all depends on what Microsoft does concerning the next Xbox, and how long Microsoft keeps the X360 afloat after the new one comes out.
For those taking history lessons, the Xbox was still selling somewhat decently when the 360 came out in the US. Sales dropped significantly, but they were still there. People were still wanting a $150 Xbox, as it was a good value. Unfortunately for Xbox sales, Microsoft cut off 100% of production of the Xbox in favor of the X360. Had they kept the production going, they could of sold more.
So ultimately, it's a question of what kind of lifespan the 360 has. There is plenty of life left for the 360. It's still $279 for the Arcade, and has plenty of price drops to go before one could say sales are going to take a significant hit.
IMO, it should get pretty close to 50m. I'd give it a 50/50 shot of breaking that number. 3 more years @ 8m/yr, would put it at 42m+, and could sell more if MS keeps it alive for a few more years.
Back from the dead, I'm afraid.