The most successful Xbox advergames to date are the Burger King games, which sold for $3.99 along with value meals. "For Burger King it was important to get people into the store" to increase revenue, said Jeff Benjamin, interactive creative director at Crispin Porter + Bogusky, Burger King's advertising agency. Meanwhile, last month's "Yaris" and the Doritos game are both free, or will be, to players using the Xbox Live service -- boosting branding if not sales.

A game is anything, but when it comes to designing one, particularly when the end-goal is branding, "gamers are very finicky," Acharya warned. "If a game's relevant, or brings value -- that is, gives them something for free -- you're a hero. If not, gamers are very vocal and will make sure everyone knows about it."

Lazzaro said that brands also must go against conventional marketing wisdom and listen to the game-making experts, even if it hurts. "Say that you want to simulate a nuclear reactor and [marketing execs] say, 'We don't want a core meltdown, we only want right things to happen with the brand.' No! A core meltdown is what makes the game fun!"

According to Associate Brand Manager Jeremy Tucker, Doritos knew all this when it kicked off its contest. "We said that the game idea didn't have to be about Doritos, it just had to be in the spirit of the brand," Tucker said. "It doesn't have to be a giant Doritos bag smacking people into a wall."

Another balancing act that branded games must accomplish: appealing to gamers both casual and hardcore. True, a well executed advergame will grab the enthusiast's attention the way, say, a brand-centric version of "Minesweeper" won't. But casual gamers make up a much broader demographic, as well as a lucrative one, with more than 200 million people playing casual internet games each month, according to the Casual Games Association.

"Ideally a game appeals to frequent gamers who show it to their more casually playing friends," Lazzaro said. "The game should offer something for both, to reach the widest number of people with the message."