FWIW, all three of the Walmarts in Columbia, MO haven't carried these in ages. I don't know if that's true elsewhere, but it sure is the case here.
I had a $80 giftcard a while ago, and figured I'd consider getting a PSP. When I realized none of them had a Go, it grabbed my interest. I tend to check out the games whenever I'm there, and to this day, I have yet to see a Go.