Here’s a somewhat arbitrary listing of stats about current and future mobile marketing and smartphone use. I add to this as I can. (Please add new comments or sources in the comments below.)

Smartphones in General

Smartphones appear to be spreading faster than any technology in human history. http://www.technologyreview.com/business/40321/?ref=rss

According to GigaOm, Smartphone sales have surpassed PC sales, worldwide. According to PC Magazine, “vendors shipped 100.9 million smartphones during the fourth quarter, according to Monday data, while IDC logged 92.1 million PC shipments during the same time period, according to IDC numbers from January.”

Although it’s expected that half the total U.S. population will own a smartphone by the end of 2011, some age groups have already surpassed that mark. Nielsen’s third-quarter metrics show that 62 percent of the 25-to-34-year-old U.S. population has a smartphone.

Slightly younger and older age groups are also beyond the 50-percent mark. But the fastest-growing age group adopting smartphones in the past quarter is a surprise: It’s the 55-to-64-year-olds getting their smartphone on.

Small businesses are increasingly reliant on mobile apps for day-to-day operations. Nearly 3/4 of small businesses report they use apps, and more than 1/3 say they’d face challenges without apps available. The most popular reasons for using apps in small businesses are time savings, increased productivity, and cost savings.

The average paid iPhone app has 100,000 downloads over an average life of 9 months.

The average smartphone user in the US is more likely to be a business owner, and on average earns twice as much non-smartphone users.

The Spanish language market is the fastest growing smartphone and tablet market in the world.

Hispanic users are the fastest growing market segment in North America, with a significantly higher existing use rate for smartphones and tablets.

About Apps in General

Americans now spend more time on Facebook mobile than its website.

The iPhone and iPad are dominating e-commerce. “At Fab.com, a flash sales site, 95 percent of its mobile visits (which make up a third of its overall visits) come from the iPhone,” Matt Lynley writes for The Business Insider. “At OneKingsLane, another designer flash sales site specializing in home decor, iPhone and iPad owners stand far above other mobile devices among mobile shoppers.”

About the Apple App Store

74% of iTunes App Store proceeds are from in-app sales. (Citation needed.)

… iOS is expected to remain the second biggest platform worldwide through 2014. (Gartner)

A variety of global app download reports are available here: http://xyologic.com/app-downloads-reports.

BloomWorlds BloomBlog reports that the average smartphone user is considerably wealthier than the average adult. Smartphone users have an average household income of $66,000. In contrast, the average household income is $44,000, meaning smartphone users are on average 50% wealthier than the general population. Smartphone users are more than 2 times as likely as the average mobile subscriber to make more than $100,000/year.

According to Mobile Entertainment:

• The average paid iPhone app has sold 101,024 copies over a lifetime of 261 days
• US iPhone users download 60 apps a year on average
• 51% of iPhone owners have five or more games installed on their devices says Compete
• 37% of iPhone owners play games at least every day—puzzle games are most popular

“The hype that surrounded Apple’s announcement about the iPhone 4S reflects not only the interest in Smartphone’s but also the demand for apps.  In a little more than 3 years, more than 18 billion apps have been downloaded from the App Store and that number currently stands at over a billion a month. This number excludes other platforms such as Android.

“The app market is one of the few sectors that remain unaffected by the current economic climate and this is drawing in more and more business people who want to channel their creativity so as to improve productivity and profitability within their organisation.

“With the app market expected to reach $25 billion by 2015, how can UK SME’s and other businesses tap into the App revolution to fuel their growth?”  — Kevin Miller, writing for Developer Tech

About Android

Android is increasingly fragmented: there are literally thousands of different Android devices (almost 4,000 in one recent count) on the market with different specs, such as screen sizes, processors, buttons, supported hardware features and more.

Older info that may still be useful:

Worldwide sales of smart phones are expected to reach a staggering 1.1BN in 2015, according to a (not so) recent report by Gartner.

Android is expected to consolidate the dominance they set in the market at the end of 2010, growing their current 23% market share to an authoritative 49% in 2015. “As vendors delivering Android-based devices continue to fight for market share, price will decrease to further benefit consumers” said Roberta Cozza, principal analyst at Gartner.  “Android’s position at the high end of the market will remain strong, but its greatest volume opportunity in the longer term will be in the mid- to low-cost smartphones, above all in emerging markets.”

Business Insider says global app downloads for year-end 2011 are expected to balloon to 29 billion, compared to only nine billion in 2010. Such stellar increases are largely due to the proliferation of smartphones around the world. The total smartphone install base is expected to grow 46% in 2011.