Sony announced at the business briefing yesterday the PSP-290 Sony PSP accessory, which is a GPS receiver.

Update:
PSP GPS released in Japan on 12/07/06. Available at Play-Asia for Order.

I updated our nearly 1 year old Sony PSP Feature Request round-up, where GPS was one of the hottest requests. Sony needs time, but they get around to release features that Sony PSP fans want to have eventually.
GameSpot has published slides from the Sony Business Briefing showing a blurry image of the Sony PSP GPS antenna. The mounting looks similar to the Sony PSP microphone.
The GPS antenna is supposed to update every second and has a 40 second cold start until fixing the satellites.
There has been already a Sony PSP Map software announced (photo). The Sony PSP-290 GPS antenna is supposed to ship in October. Pricing is still open.
More details in this Sony press-release (Japanese).