If you look in the Files section of the Yahoo SDM Group, you will see a circuit that I built from a 5V DIL relay and a couple of components.
After your remote switch is released, the drop-out for the relay will depend on its coil resistance and the size of the low-leakage tantalum capacitor.
I found it works very well.
HOWEVER, someone has designed a commercial USB-remote switch that includes triggering external flashes, expansion to as many cameras as you wish, synchronised time-lapse and provision for external power for long-duration time-lapse.
The PCB design has been sent away for manufacture.