As was trumpeted across the Internet Monday, Google (GOOG) is buying mobile display advertising startup AdMob for $750 million in (increasingly) precious Google stock. Wall Street digested the news and sent Google stock up almost $11.

Citi analyst Mark Mahaney says the deal “makes sense, because Google is moving aggressively to take advantage of the strong growth opportunity in mobile, which is fueled by smartphones.” Sandeep Aggarwal at Collins Stewart likes the deal, arguing “mobile advertising will be a $4 billion revenue opportunity by 2012-2013.”

Over my dead BlackBerry.

OK, I am in agreement that the whole smartphone movement is big, really big. But the ads on them? They are small, really small. In its own FAQ on the deal, Google brass acknowledge that mobile advertising is pint-sized today. They cite a number from eMarketer that pegs spending on mobile advertising at $416 million in 2009. That compares to the nearly $24 billion spent overall on online advertising.

It is true that $416 million ain’t chump-change, but it’s not Google dollars either. Estimates for AdMob’s gross revenue are in the neighborhood of $50 to 75 million, with a net of around $20 million. That is tiny, but presumably it will grow fast once AdMob’s display ads and universe of publishers and advertisers can plug into Google’s AdSense. But let’s get back to that small thing.

If you think online display ads are at best an annoyance on a 30-inch monitor, what about a three-inch screen? Ignoring ads on a PC is easy enough; on something I pay $60 or $80 a month for (especially if serving up the mobile ads slow my wireless network even more) ignoring the ads will be the default mode. Yes, there will be location-based bells and whistles to go along with the mobile ads — 30% off a ham sandwich and shoe-repair 30 feet from where you are standing — but that is still a ways off, and do you really want mobile coupons? Mobile advertising has been one of those things that gets promised year after year, and never seems to quite materialize (sort of like true broadband in the United States).
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The mobile Internet is happening, and fast, the iPhone has shown us that. Whether an advertising experience works well enough on smartphones to really move the needle (and not simply cannibalize the non-mobile online ad world) remains to be seen.

Google: Buy vs. build

And by the way, Google knows how big this mobile Internet thing is going to be. Why couldn’t they figure it out, and save the $750 million? They have buildings filled with very smart people, and a good culture of “rolling their own,” as it were. I guess if your stock is up 83% since the beginning of the year you don’t have to sweat that too much.

Respect to AdMob for getting this deal done (barring any regulatory issues). Big ups to Sequoia Capital, Accel Partners and Draper Fisher Jurvetson who will make a bundle on the acquisition (and LiveOps CEO Maynard Webb who invested his own eBay money in AdMob). AdMob is clearly running fastest in this new mobile advertising world, and Google has the money to pull them off the startup track and install their 140 employees at the Googleplex. It seems there are plenty of Odwalla smoothies to go around, but I wonder, when will the big mobile advertising dollars arrive?

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By Jon Fortt, senior writer

AdMob CEO Omar Hamoui doesn’t take himself too seriously. When I had lunch with him recently and he eyed the gourmet burgers on the menu, he had no qualms about asking one of his employees what aioli is. (It’s garlic mayonnaise.)

He shrugged and explained that he usually grabs a cheap sandwich, so he’s not used to the cloth napkin fare.

Hamoui’s easygoing manner shows in other areas. He doesn’t have an office in AdMob’s modest San Mateo headquarters — in fact, he doesn’t even have a cubicle. He works at one end of a row of computers, shoulder-to-shoulder with other engineers.

As you’ve read above, my colleague Michael Copeland is a bit down on Google’s decision to purchase AdMob for $750 million. Let me quickly offer another view. I think the AdMob deal is both a great business move and a signal that Hamoui can still fit in at Google — because the search giant is levelheaded enough to swallow its pride when it matters.

Let me explain. AdMob’s approach to mobile advertising is starkly different from Google’s. While Google has tried to squeeze its wildly successful PC-centric advertising onto the phone, AdMob has built a custom system that treats the phone as a unique sort of device. The differences between Google and AdMob were more than academic; they sometimes led to flare-ups between two passionate competitors. (Kind of like Copeland and me.)

A couple of sparks actually flew at Fortune’s Brainstorm Tech conference this past July. During my “Future of Mobile” breakfast roundtable, Google engineering VP Vic Gundotra argued that Google’s approach to mobile ads was superior. Hamoui listened quietly before offering a rebuttal. But in the audience, under his breath, AdMob executive Jason Spero used some colorful language to inform his neighbors that Google was full of it.

Right after breakfast, Gundotra confronted Spero. A Google employee nearby had picked up Spero’s comments on her audio recorder, he said — and he casually suggested that if the recording ended up online, it could make AdMob look pretty bad. The clear implication: Watch what you say about us.

Less than four months later, bygones are bygones. Google executives realized AdMob is better positioned in the must-win mobile market, and decided to pay up before AdMob gets even more expensive — or worse, gets acquired by Microsoft (MSFT) or Yahoo (YHOO).

Sure, you could look at this as evidence of a problem. You could argue that Google has an expensive habit of failing to build the best products in new markets like online video and mobile ads, and getting outsmarted by spry startups.

But here’s another take: Google knows how to pick its battles. CEO Eric Schmidt has openly declared that mobile advertising is a key piece of his growth strategy, with the potential to be as big as Google’s core PC-based business. If Schmidt believes that, it doesn’t matter that AdMob’s revenues are small today — what matters is that AdMob has the right people and the right technology to win in mobile.