I was in Sydney last night for Earth Hour and the difference from the usual city lights was impressive. This exercise is certainly helpful in reminding people of the issue.
On the other hand, I think the implied message of virtuous self-denial is the wrong one. A typical household would save more CO2 emissions by laying out a few dollars to replace one incandescent light bulb with a compact fluorescent than by turning everything off for Earth Hour once a year, not to mention coming out ahead financially. And what’s true for lighting is true for consumption in general. Efficiency improvements and substitution (videoconferencing for business travel, for example) can do a lot more than any plausible reduction in living standards.