I think that someone else will sponsor it, however, if not, I think Disney will continue to operate it until a sponsor can be found (once the economy is better perhaps). This is too popular of a ride to shut down.
It's a great ride, but I'm not sure any car companies are in a place to take over sponsorship. Hopefully someone will save the day like Siemens did with Spaceship Earth, but who knows...
I've always wondered why Disney doesn't just sponsor the good rides themselves... ?
The news in Orlando reported that Toyota has an interest in picking up the sponorship of the ride. I am sure Test Track will still be there one way or another.