Bloomberg's face is prominent in the ad, where he's joined by other members of Mayors Against Illegal Guns, which he co-chairs. The ad, now running in the Des Moines Register and the New Hampshire Union Leader, urges both candidates and voters to focus on the spread of illegal firearms, and challenges those seeking the White House to address the issue.
If Bloomberg were to run, he would do so as an independent, so it's not as though he could have any impact on the Iowa caucuses and the New Hampshire primary. But his mere appearance in newspapers in both states -- in an ad he was presumably OK with -- is going to remind people of his presence just when the election is heating up and draw national media attention.
Reports indicate that Bloomberg won't decide on running until after the nominees of both parties are known, a circumstance that is likely only about a month away. In other words, now is a pretty good time for Bloomberg to be in the spotlight -- something the new ad could accomplish.