actually, the NUMMI (New United Motor Manufacturing, Inc.) plant in Freemont, CA has been in operation since Dec. 1984. The web addy is www.nummi.com and the solid address is:
New United Motor Manufacturing, Inc.
45500 Fremont Boulevard
Fremont, CA 94538
Toyota and GM have made several vehicles there, starting with the Corolla/Nova, then the Corolla/Prism, then the Matrix/Vibe. Toyota has also produced the Voltz, a low production vehicle only sold in Japan, and the Hilux pickup that evolved into the Tacoma.
The Tacoma, Matrix and Vibe are currently built there and it has been a great deal for both companies. In the early days, NUMMI gave Toyota a strategic manufacturing location in the US and GM got access to good, small vehicles that they didn't have to invent from scratch.
Now, Toyota has eleven major manufacturing facilities in North America and Canada and dozens of smaller supplier locations, so NUMMI has become just one way for Toyota to meet the demand for Corollas and Tacomas.
The Matrix and Vibe, while built on the same base platform, are not the same vehicle. The Matrix has thicker glass, more insulation and sound deadening, is offered in three trim levels, is offered with four-wheel drive, and has more extensive and more advanced safety features (four-channel ABS that controls each wheel independently versus GM's two-channel ABS, for example). For 2008, GM only offers the Vibe in one base trim level and only in front wheel drive.
The Matrix is based on the Corolla platform and the Corolla is being completely redone for 2008, so GM may or may not have a vehicle based off the new Corolla. Seeing how GM has pulled back on how the Vibe is offered, I would expect them to wait and offer a totally different vehicle once the new Corolla is out. I would expect that new vehicle to be some sort of crossover/wagon type of small car, looking at current trends.
I teach about hybrids and advanced tech for Toyota.