It depends if you count the inevitable PSP phone Sony is going to release to compete with the iPhone. ;)
Leaving the phone aside, I think it may reach up to 80 millions, if the current support continues. However, I would not be surprised to see a PS2P sooner rather than later, let's say 2011. If Sony brings back PS2 BC to the PS3, and brings PS2 classics to the PSN, I'm expecting to see a PS2P with BC to PSOne, PS2 and PSP.
For me, the PSP Go is mostly about trying out a full digital distribution model.







