The only standard available at the time of this writing that one could compare the first week of sales for the PSP go! would be to compare it to the first week of sales for other hand held gaming devices.
The original PSP
The Nintendo DS
The Nintendo Gameboy Advance
The Nintendo Gameboy Color
Some may claim that, as a new product which lacks the traditional retail channel it would be unfair to compare it to said consoles, but it is the only standard we have to go by for at the moment.
But; it could also be argued that comparing it to the original PSP would be fair as when it debuted it was opening with a new media standard (UMD) to play games/videos.
Also, when more information becomes available we could judge from our POV how bad/well it did according to the expectations of SONY, since we don't have hard numbers to go by at the moment, things to take into account, like : costs of development, costs of manufacture, and profit/loss in sales.