It seems that at least Japanese PSP owners will be able to buy a GPS system for their PSP called the MAPLUS portable Nabi shortly. The GPS turns the PSP into a full navigation system with a moving map.

It seems the entire country of Japan is mapped in the GPS software and the map is updated in real time so you should be able to get turn by turn instructions as well. I will be the first to say Google Translations are sketchy, but it seems the image you see here is a mock up and not what the actual product will look like.
The maps look pretty cool and you can get both the wide view map along with a split screen of the local turn by turn directions displayed on your PSP at once. No word on if the product will be available in the USA. The launch date is set for mid-December and the price is not announced at this time.
The Sony PSP GPS hardware accessory has been announced in Japan a month ago.
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