I'm not aware of any SART north of the dam. It is possible there are some short sections. If you can find the book "Bicycle Rides - Inland Empire" by Don and Sharron Brundige it might have some other river sections. I don't think there is a continuous trail that is easily followed.
I'll look around to see if I can find any maps of trails in the inland empire.
Update. Take a look at this website. http://www.mapmyride.com/ride/united-states/ca/san-bernardino/289413464
It should show a google map with a 21 mile trail that goes from Tequesquito Arroyo Park in Riverside to Waterman Ave in San Bernardino.
Also found this one. It seems to connect the two sections of the SART. http://www.mapmyride.com/ride/united-states/ca/san-bernardino/705029175
Update 10/23/09 I just rode by the SART at Gypsum Canyon Road. When you come up to the bridge you will notice that the center lines direct you either to the right (and under the bridge) or go straight seemingly to the intersection. As you pass the chain link fence and gate you will see a small sign on the bridge indicating the trail goes right and over the bridge (and the Santa Ana River). There is also a Bike Route sign on the first light pole.