Yep, through looking around 600 seems to be the magic number. Haven't done it much myself either, but in case it's helpful for you this is an example of my first try tonight before I saw this thread:
30sec @ 30mm, ISO 400, wide open on my 17-40 (so f/4)
The stars appear to be pretty circular and "frozen", but a crop of the lower left corner reveals that at 30mm, 30sec could be just a bit too long (just as the "600 rule" suggests):
And in retrospect, as the advice here has said, a higher ISO seems important. Same shot at 3200 would probably be a night and day difference. I was worried about noise, so I kept it low, but it's not much of a picture because it's definitely underexposed.
Hope that helps.
The other thing to consider with star trailing is where in the sky you photograph, towards the equator the stars will trail quicker then shooting towards the north star.