Ed,
I know where you are coming from, having faced the same problem. I used to shoot 4-5 hour star trail exposures in slide film, no problem. With digital, it's a different story. Does not mean it is worse or bad, just different. Actually, in some instances is better, since there will be no reciprocity failure, for example.
I shoot nightscapes with my 5D MKII and 24 L MKII (I prefer this lens, because the fast aperture helps in collecting more light and in capturing the fainter stars). Normally, I am using ISO 800, 30 seconds, and f/2, or f/2.8. I have't tried star trails yet, but it seems relatively easy to shoot with an intervalometer and stack the exposures later. Make sure you take a few dark frames, there are softwares that will use them in the stacking process to abstract noise (the already mentioned startrails.de is one I am planning to use).