There are some interesting statistics in this July 2009 Metrics Report from AdMob, including:

• Android and iPhone users download approximately 10 new apps a month, while iPod touch owners download an average of 18 per month
• Upgrading from the lite version was the top reason given when users were asked what drives them to purchase a paid app
• iPhone and iPod touch users are twice as likely to purchase paid apps than Android users. Users who regularly download paid apps spend approximately $9 on an average of five paid downloads per month