Wi-Fi usage has taken off, with 24 percent of requests in the US coming in over a WiFi network in November 2009, compared to 8 percent in November 2008. Other WiFi devices, such as the iPod touch, Sony PlayStation Portable (PSP), and Nintendo DSi also gained traction in 2009.

Highlights from the November 2009 AdMob Mobile Metrics Report include:

• 55 percent of ad requests in the US came from devices with WiFi capability, up from only 19 percent a year earlier.
• The Top 5 US devices based on WiFi requests generated were the iPod touch, iPhone, Sony PSP, HTC Dream (G1), and Motorola Droid.
• 36 percent of iPhone traffic in the US was over WiFi, considerably higher than other WiFi capable devices. Less than 10 percent of traffic from the major Android devices came over WiFi.
• The iPhone accounted for 71 percent and the iPod touch accounted for 29 percent of total unique users from Apple devices.
• Android generated 27 percent of the requests from smartphones in the US in November 2009, up from 20 percent in October 2009.
• 88 percent of requests from Android devices came from the US in November 2009, the second largest Android market is the UK with four percent of requests.

From: http://www.dailywireless.org/2009/12/22/smartphone-wifi-major-trend/