Interest in mobile advertising is up, but inventory is not be keeping pace with demand, a trend that may only increase as we head towards a recession. Already, evidence is surfacing that big brands are cutting back on their mobile ad buys, which will make AdMob’s job more difficult since it already doesn’t have enough ad inventory to fill every request coming from publishers. AdMob has been closing the gap slowly, but it’s still not able to fill all of its ad requests. For instance, in the U.S., AdMob served 1.79 billion ads last month, but had requests for 1.99 billion, and globally, it served 4.53 billion ads compared to 5.07 billion requests. As a caveat, AdMob said sometimes it chooses not to send ads. “AdMob also has certain minimum performance thresholds and limits ads served to low-performing inventory, which also can negatively affect fill rate,” a spokesman said.

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Despite this gap, AdMob’s inventory is still growing month-over-month, and it is serving three times as many ads today than it did a year ago. That may give it a chance to catch-up as demand tapers. On a global basis, it served 21 percent more ads in September compared to the prior month, and yet ad requests from publishers looking to monetize their mobile offerings were essentially flat, dropping .8 percent.

However, there’s fear that there will be fewer ads to serve going forward. Some brands are cutting so-called “experimental” ad budgets, which would include mobile. Chrysler, which is being hit especially hard from the widening economic disaster, has cut mobile and other new media ad buys in half, WSJ reports. Even with more frequent reporting coming out, the data just isn’t sufficient enough to justify ad spends for some companies, particularly those desperately needing advertising that works and translates into sales. “We won’t experiment in a lot of things that are fun to have. All of our dollars have to go to hitting in-market shoppers with the appropriate media,” Chrysler’s CMO Debarah Meyer told WSJ. If enough companies follow suit, mobile ad networks’ inventories are likely to decline, potentially not only affecting AdMob’s statistics, but also heavily funded startups that were expected to make money on mobile advertising. What’s more new investments might be even harder to find, which could open the door on a wave of consolidation or shutdowns.

More highlights from AdMob’s monthly report after the jump…