SEGA's iPhone version of Super Monkey Ball sold more than 300,000 copies in its first 20 days on sale, the Wall Street Journal reports.
Simon Jeffery, president of SEGA of America, described the iTunes App Store, which sells the tilt-sensitive puzzle game, as "a substantial business".
"It gives iPhone a justifiable claim to being a viable gaming platform," he told the WSJ.
The App Store, which launched just over a month ago, allows iPhone owners (and iPod touch users with 2.0 firmware) to buy and download applications in the same way they currently acquire music and TV shows from the iTunes Music Store.
Early leaders in the games section include Super Monkey Ball, obviously, as well as Crash Nitro Kart and Bejeweled 2.
Apple acts as gatekeeper for content, judging what to allow, and has already yanked a couple of programs - including one called "I Am Rich", which displayed a red gem and cost USD 999.99.
Things are going well otherwise, though, as occasionally beardy Apple boss Steve Jobs confirmed to the WSJ. "Who knows, maybe it will be a USD 1 billion marketplace at some point in time," he remarked, as the newspaper reported his prediction that sales would hit USD 360 million if they maintain their current pace to the end of the year.
Look out for that oft-promised iPhone coverage of ours very soon.