As was the case with the DS Lite and DSi, when the PSP Go arrives in October it will come with a variety of new accessories that are not compatible with the handheld's previous incarnation.
With the DS Lite, for comparison's sake, the AC adapter had a different proprietary connector, which was a nuisance if you lost it and happened to still have your original DS adapter handy.
For the PSP Go, there's this picture, which shows the range of accessories that mate exclusively with the new handheld's multi-use port. The old PSP used a mini-USB port. You get the picture.
Well, strike that, actually. With a little prodding and stretching, the pouch could definitely work for both models. [Joystiq]