I'm still pretty new at this having similarly just bought a 650d, but I have already come up with my own simplified equation for exposure without star trails. Its easier to use that the others and much along with Gregs comments. Its:
30 seconds or less / conditions on the night.
The attached photos are using an 8mm lens and with a 30 second exposure. Everything looks great zoomed out, but I chose an area with the LEAST trailing to enlarge.
I have been reading the Nightscapes section avidly, and that seems to be the general concensus of exposure before having to resort to making a composite image with the stars tracked.
The 600 rule gives 8mm @ f4.5 = 16 seconds which is better but will require higher ISO and corresponding noise issues.
I haven't done Joes' maths, but of course the key factors are fl, pixel size (chip size is sort of irrelevant) and rate of movement of the field. I'll stick to my equation for the timebeing
