For one, Tesla has no plans to deliver cars with transmissions that it knows will fail. Instead, the plan is to deliver some cars with a simpler, single-speed transmission rather than the two-speed gearbox being developed. Tesla assured ABG that this single-speed tranny will meet all durability requirements and not leave owners stranded on the side of the road. So these transmission are temporary in the sense that they'll eventually be replaced, not that they'll fail.
The single-speed trannies, however, won't allow the cars to go as fast as promised. The two-speed gearbox allows the Tesla Roadster to meet its goals of a 4-second run to 60 mph and a top speed of around 125 mph at the same time. The single-speed gearbox, however, will be trading off some low-end acceleration so that the car's can be delivered on time.