A while ago I disagreed with a column penned by Jay Michaelsen in The Forward. Now I want to agree with one. This time, he argues for observing the Shabbat according to season, i.e. early on Friday in the winter, late into Saturday in the summer. Actually, I wouldn't have thought there is any other way possible, but obviously I'm uninformed on that. Anyway, Michaelsen's column contains all sorts of interesting points, about Shabbat, about observance in general, and about belief vs. practice.
Btw, the whole issue is an American vs. Israeli one. In America, the surrounding society doesn't have Shabbat, so observing it at its extremes causes problems, hence the need to tinker with its schedules. In Israel, whether you're orthodox or not is irrelevant: The Shabbat starts according to sunset. Just to give an example: an army unit sending its soldiers home for Shabbat has to allow them to get there before sundown, be that 4pm or 8pm, and even if all the soldiers in the unit are Druze.