For all the talk about Android growing faster than the iPhone, Apple’s device remains the top dog in usage – for now. According to the latest “Mobile Mix” mobile device index from Millennial Media, Apple was the leading device manufacturer, top phone and top OS in terms of ad impressions on Millennial’s network in May. This continued supremacy can be attributed in large part to the fact that there are still more Apple OS than Android devices in-market.

On the other hand, Millennial points out that overall Apple ad impressions have decreased 32% since the beginning of the year. In the same time frame, Android impressions have surged 338%. This reflects Android’s upward trajectory, spurred by the launch of  new Android devices on all the major carriers.

Other key findings include:

• Apple ad requests declined 33% month-over-month, while Android requests grew 15%
• RIM remained the second largest OS on Millennial’s network for the 10th consecutive month, and had five BlackBerry devices in the Top 20 for the second consecutive month
• BlackBerry ad requests saw a slight month-over-month decline, but have grown 33% overall since January

As Greg Sterling observes, these trends are consistent with what we’ve seen from other ad networks, including AdMob, in its April Mobile Metrics Report, and firms such as NPD Group, which track handset sales data. The bright spot for Apple remains the iPad, which saw a 160% month-over-month leap in ad requests on Millennial’s network.

Strong marketing support for Android, from the carriers and device manufacturers alike, will help propel Android forward. That said, judging by the healthy demand, to put it mildly, for the iPhone 4 (600,000 pre-orders on its first day alone), the battle is far from over.

Either way, marketers stand to benefit. The ever-increasing competition between the two companies is contributing to the sizable 43% growth in mobile ad spending this year, and a 37% CAGR from 2008 to 2013.

Google announced today it is buying leading mobile ad network AdMob for $750 million. That’s a hefty sum for a medium still considered to be emerging, although it’s still half of what Google paid for YouTube and pocket change for a company with as big a war chest as Google’s. Then again, Google tends to put its money where the market is (or, alternately, the market goes where Google puts its money, even if in the case of YouTube, ad dollars didn’t follow as quickly as Google would have liked).

Either way, Google’s acquisition sends a strong signal about the importance of mobile advertising in general and its growing centrality to the marketing mix. The move also indicates that as on the desktop, the twin bill of search and display will be the big driver of ad spending on mobile as well, a trend I identified in my recent “Mobile Advertising and Marketing: Change Is in the Air” report (full version of this report is available here to eMarketer Total Access subscribers only).

In the report, I predict that US mobile display spending will grow more than fivefold between 2009 and 2013 and mobile search will increase sevenfold. Looking at it another way, search and display represent 45% of US mobile ad spending in 2009, but by 2013, they’ll account for 72%. Key factors behind these big shifts are growing smartphone penetration and increased Web and application usage, as indicated in the following report excerpt:

A number of factors likewise point to steeper growth for display advertising. One is greater use of the mobile Web. eMarketer predicts that 36% of mobile subscribers will be accessing the Internet from their devices at least monthly by 2011, up from 26% in 2009. More surfing means more exposure to ads.

Another factor is increased usage of mobile applications, specifically free, ad-supported apps. All the available data suggests they will continue to become a more central facet of the mobile ecosystem, meaning that in-application display advertising will likewise grow in importance.

A third factor is the increasing array of attractive inventory options available to marketers. These include interstitials, takeovers and rich media interactive ads.

With respect to search, the logic behind predictions for its increasing primacy is clear enough: On the desktop Web, search ranks as one of the top online activities. As mobile devices mimic desktop PCs more in their capabilities, search will grow in importance on the mobile Web as well.

The AdMob acquisition comes at a time of renewed (or perhaps redoubled) marketer enthusiasm for mobile. Millennial Media, another leading mobile ad network, recently released the findings of its “state of the industry” study, conducted with online knowledge base DM2PRO, which indicated two key trends for 2010: more marketers planning on employing mobile advertising in the year ahead, and bigger budgets, especially among the brand marketers surveyed. All told, good news for the mobile ecosystem, especially for ad networks, which are now in play even more than before.