YouTube has started testing out display ads for its mobile version, m.youtube.com. The company has been developing a mobile interface over the last year, and now says in a blog post that “people watch hundreds of millions of YouTube videos every month on mobile devices,” though I would guess a majority of those are on iPhone, where it is a natively-resident app…few other handsets can support streaming of Flash video.

SEE ALSO: YouTube Expands Mobile Access: Opens Video Library, Upload Tools To Majority Of 3G Handsets

On the ad test, it has started running a test of display ads on select pages of the YouTube mobile site in the U.S. and Japan. “This is our first step in testing mobile advertising for YouTube—it will give you a new way to interact with content on the go, while allowing us to learn how video viewers engage with mobile advertising. Our test advertisers will also have an additional branding tool at their disposal and the opportunity to reach the millions of people who visit YouTube every day on their phones.”

YouTube started opening up access to its mobile version early this year, when it broadened its mobile service to include most of its video library and upload capabilities to a majority of mobile devices with streaming ability and 3G access.