Spartacus:
The SBA dollars, the Sacawagea dollars, and the Presidential dollars are all the same size, weight, and composition. A properly calibrated vending machine, fare box, etc., should accept them all interchangeably. Only the color and design is different.
Also, legislation signed earlier this year should result in new Sacawagea dollars being released for circulation in 2008, and in 2009 with a new design on the reverse side.