I think Sony's prepping another revision of the hardware and maybe a new store model (Layout of games in stores and such maybe a whole new Greatest Hits line). We'll find out next week at TGS. On a related note the video rental store up here in Canada, a pretty big one, cleared out everything PSP for half price.
PSP4000's a dead cert imo. I think that's their holding plan for the next year, as well as a renewed focus on DD releases (though without Go as the centerpiece).