VG247's reporting that Battlefield Heroes won't have any in-game ads, and will be funded instead from ads on the official site the game launches from.
From the piece:
"Cousins explained that the game would depend entirely on the website adverts that players saw when launching the game, and during loading. "We won't be putting billboards in the game," he said, as the team wanted to preserve the clean, cartoonish world they've created. The crowd murmured as the game loaded and we were stuck looking at an advert for Crysis. "We've been known for our long loading times," Cousins jested. "Don't worry, we're going to optimise.""