[to anybody coming here from the no_lj_ads post - hi! The vast majority of my journal is friends-only, so you won't see a lot of public posts from me here. I generally only post publically when there's an issue that I want to vent about that I feel everybody needs to see - like this. Feel free to add me if you like, but I probably won't add you back unless I know you.]
[Update: it looks like my post to no_lj_ads has hit Slashdot.]
Okay, I will. It ain't so. There's been no LiveJournal policy change. Popups are not, as far as I know, ever going to appear on LJ.
So why are some S+ users getting them now?
To cut a long story short, this is because of a certain advertiser - kpremium.com (not linked; don't go there, I don't believe they deserve it) - being sneaky and underhanded. It's not LJ's fault, and I have no doubt that the ad will be gone in a few hours.
The ad itself is for a program that lets you download stuff - you know the sort of thing. The ad is a Flash ad, and masquerades as a banner ad.
Thing is, the Flash ad contains code to open a popup that leads to a very different destination - it's what I assume is an affiliate link that attempts to download and install ErrorSafe on your computer (link is to Symantec's description of it).
This, of course, would be totally against any ad company's guidelines. Masquerading as a banner ad, but discreetly opening a popup - and not only that, but to what people consider malware - is totally against any ad company's guidelines. So how did it get through?
Simple - the ad actually contacts its website in the background, and the site returns a response code that tells it whether to display the popup or not - "popup=1". My guess is that kpremium.com returned "popup=0" while the ad company were testing the ad for conformance to guidelines, and then they turned it back on once it was out in the wild.
This, my friends, is an absolutely despicable way to do things.
So, it's not LiveJournal's fault. No LJ policy change has been made. kpremium.com are the underhanded ones, and the ad company didn't spot the request the ad made. I have every faith that LiveJournal will nuke the offending ad as soon as possible.