Is there a way to get the bulb mode on the D90 to open the shutter when I press the shutter release button, and to close it again when I press the button for the second time, rather than having to hold the button down ?
If you get the Nikon MC-DC2 cable release, the button can be locked in place until you release it. For bulb exposures, I'd recommend a cable release to avoid shake and blurring in your result anyways, you're almost assured to do that with your hand on the camera directly.
Wireless one is good too.
When i bought it, it was actually cheaper than the wired one. Gotta aim it directly at the camera front though.