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.
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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]