However, the USB port does not provide a way to open and close the shutter on "bulb."

Accordingly, if you want to make long exposures under computer control, you'll need a serial-port or parallel-port cable release such as I am about to describe. Other designs are given on the web site of DSLR Focus, which is a software package for DSLR astrophotography.

The idea is to use the DTR line of your serial port to actuate the camera. DTR is high (about +6V) whenever the serial port is open for communication, and low (-6V) at all other times.

The traditional circuit looks like this:

It can be made with Radio Shack parts but does not isolate the grounds of the camera and the computer. Isolation is desirable if the camera is powered from the AC adapter rather than its own battery. Accordingly, I designed and built an optoisolated version:

It's built into the plug like this:

Note that the pins on the 9-pin serial port connector are labeled: