According to Walmart, Gran Turismo for PSP will ship on 02/01/2006.
Lets hope this is true, considering the release date was just changed within the last day or so (was originally schedueled for a Q2 2006 release).
"With authentic cars, courses, and car physics, Gran Turismo 4 adds an even more realistic racing simulation to the series. Test-drive new vehicles in courses located in various sites, such as New York City, the Grand Canyon, and Tsukuba Circuit. The PSP version of Gran Turismo 4 lets you race against your friends via wireless network."
Source