There are a couple of ways to get a single image of star trails. You can either take seperate 30 second exposures over a 3 hour period (you would end up with 300-400 images) and then blend them together with a program called StarTrails. Or you can use bulb mode and leave expose a single shot over a 3 hour period. Both methods have their pros and cons; If you are taking seperate shots you can see the results so exposure will be easier to get correct. On the other hand it means more post production. If you are just shooting a single image you get what you want straight away and minimal post production is necessary, but if you expose it wrong you have to do it all over again. The noise is also much higher in a single exposure.
In order to do my time lapse of star trails I will be taking seperate 30 second shots and then render them together with a video program, such as Quicktime Player Pro 7.
I will most likely be shooting my star trails at the burnt out Stomlo observatory, as I like the atmosphere there and I want my foreground to relate to the stars.
291, 30 second stacked exposures
500, 30 second stacked exposures
360, 30 second stacked exposures
3 hour, single exposure
3 hours, single exposure
30 minute, single exposure
Good tutorials and resources: