A WebmasterWorld member says that when he changed his meta description, his rankings plummeted on his site.
In the past, US courts found META tags immaterial but Google has recommended it. Tedster points to previous WebmasterWorld coverage that shows that there are "indirect ranking effects from the meta description, including lower clickthrough rates that can cause a decent ranking to fall away if the url doesn't perform." Barry agrees as well.
Is Google using clickthroughs as a ranking method? Forum members are not entirely sure, but one points out that Wikipedia has no meta description -- which may count for something (avoiding that lower CTR, perhaps?)
Forum discussion continues at WebmasterWorld.
