Yes, it's opt-in. Except that if you're a free user who has opted out and you click to the journal of someone who has opted in, then you see their ads. So it's not really fully opt-out after all, unless you pay them money.
More to the point, however, as I commented at the link above:
I have a permanent account so I'll never have to see the ads, but I still hate this idea. Merely being able to avoid seeing the ads does not address my objection to ads. Allowing ads puts LiveJournal in a position of striving to keep advertisers happy. Allowing ads grants certain companies an implicit endorsement from LiveJournal. Allowing ads says something about what LiveJournal is, that merely collecting money from individual users who do not exist solely for the purpose of selling products does not do. Allowing ads dirties LiveJournal's image, and I don't approve of that one bit.Also, LiveJournal is now asking people what specific kinds of ads they would most object to seeing on LiveJournal. Most people's responses seem to pertain to the layout of the ads - how big they are, whether they're animated, and so on. Mine pertains to the products being advertised. Like, if LiveJournal were plastered with ads for crafts handmade by its users? I would really not mind. But if LiveJournal is plastered with ads for weight-loss programs and those stupid personal-ad websites of which there's evidently not a single one in existence that permits one to express an interest in more than one gender without any interest in (or tolerance for wading through) the heterosexual members of either gender?
I have a permanent account. But I see major potential here for LiveJournal to become an embarrassment for me to feel associated with. NOT HAPPY. Please, follow the links and urge them at least to avoid any type of ads you can think of that could be potentially offensive, exploitative, and so on.