Don't think you can on a 350D without using something like DSUSB from Shoestring Astronomy. Newer cameras from the 40D and 450D (I think) don't need the DSUSB but earlier do.
I was going to mention that you could use timer remotes like those above, or a cheap eBay one like I have, but I haven't used it with Nebulosity. They're handy to have regardless for long exposures for things other than astro.
I have a shutter release to use with my 350D which will allow for exposures longer than 30 seconds. If I am running the camera through nebulosity, will the external shutter release override nebulosity and allow for captures of longer than 30 seconds?