Starting small, Time Inc. titles incorporate ads in previously off-limits covers

TIME magazine and Sports Illustrated have opened the door a crack to advertising on covers, with a discreet ad link for Verizon on the address label of this week's issue and next. Newsstand issues will have the ad next to the bar code. A story in Ad Age notes

The ads are tiny, but their arrival puts a big crack in the longstanding tradition that kept ads off magazine covers. 

The industry's major players have until now almost entirely resisted pressure to sell cover ads, despite a strong need for new revenue in recent years. That's partly because ads on covers violate widely-observed guidelines from the American Society of Magazine Editors, but also because most editors believed in those guidelines, which are meant to emphasize and protect editorial independence from marketers.

Advertisers are also being pitched for ads across the bottom of magazine covers (colloquially called "zippers") and possible "native" placement of ads on the contents pages. 

The Canadian Magazine Code of Reader & Advertiser Engagement  says

No advertisement may be promoted on the cover of the magazine or included in the editorial table of contents, unless it involves an editorially directed contest, promotion or sponsored one-off editorial extra (see “Sponsorship”).