Yeah, I agree with both of you. It's pretty much impossible that most non-watchers wouldn't have heard anything about new characters or Dharma or this other stuff. Just being around family or out in public on Thursday mornings without hearing about last night's episode of Lost is like trying to avoid spoilers for the new Harry Potter book the day after it comes out (though it can be done, if one gives up the Internet and going outdoors until you've finished reading the book). Of course some people may have groups of friends, family, and co-workers that don't watch the show, but then they have likely seen commercials for upcoming episodes, information about it online, or even just the cover of TV Guide.
But then you do have some people who choose to remain oblivious to this sort of thing. In Gen. TV & Media I have seen a couple of people bring up Lost who clearly have no knowledge of what the show is really about. However, these are usually people who automatically disliked the show because of it's popularity and have been avoiding information about it for that reason, so they aren't likely to start watching the syndicated episodes anyway.
But, I still think that the commercials for the syndicated show should only show clips from the most recent syndicated episode aired, like the new episodes do. Friends does a good job of this - you will see commercials for the upcoming syndicated episode containing only clips from past episodes and the one about to air. As for advertising watching the syndicated episodes in general, I really do believe they should stick to clips of the first one or two, maybe three episodes. That's what got us original viewers hooked in the first place, isn't it? I only needed to see previews of the pilot and I knew I wanted to start watching it.