One thing we can learn from YouTube's entry into the mobile advertising business: It's still really, really early in the mobile advertising business.

Why do we say that? Because we couldn't help but feel some nostalgia for the early days of the Web when we used our iPhone to check out the sample ad Google (GOOG) placed on YouTube for Mobile Monday.

What was it? Not some new-fangled, interactive, GPS-location-based, behaviorally-targeted, local coupon ad, or a video ad hovering over a YouTube video, or anything like that. Just a small DoubleClick banner ad for Mountain Dew. And, on a less-sophisticated phone we tested the site on, a text link.

The good news for Google: It's still really, really early in the mobile advertising business. We're not sure how many people will end up seeing this ad -- Google boasts tha "millions of people" visit the site on their phones a day, but that seems high to us. But in any case, if Google is ever going to make a multi-billion-dollar market in mobile advertising, it's a good first step toward getting users comfortable seeing ads on their phones -- and toward developing smarter, higher-tech, higher-yield mobile ads.

See Also:
Google Still Dreaming Of $50 Billion Mobile Ad Market
More Ads Coming To iPhone Apps: AdMob Expands iPhone Ad Network
iPhone App Advertising: Barely There, But Interesting
Coming Soon To Your Cellphone: More Ads