PSP Go isn't going to replace the PSP-3000. In my opinion, it's a test by Sony to see how viable a handheld console is that relies on downloads for content, while also trying to appeal to those who think the PSP is too big (length and width, not depth).
I don't know if anyone realized this yet, but the PSP Go has a smaller screen than the regular PSP. 3.8" vs 4.3". Pretty substantial difference.







