The November AdMob metrics report is out and it features data on the rise of WiFi powered devices or WiFi networks as mobile Internet access points. According to the findings, in addition to the iPhone and other WiFi-enabled phones, "mobile devices that aren't phones (e.g. Sony PSP, iPod Touch) are also generating significant traffic on WiFi."

Note below that PND Dash is one of the leading WiFi-enabled devices in the US market. 

Here are the top-level data on WiFi devices for the US:

• 8% of total US requests in November were on WiFi networks, up from 3% in August.
• 19% of US requests came from WiFi capable phones, up from 9% three months prior. 42% of iPhone requests are made on WiFi, notably higher than most other WiFi capable phones which average between 10-20%. iPhone WiFi usage is generally higher on iPhone specific sites and applications than on normal mobile web sites.
• Most HTC devices are WiFi capable, with 9% of all HTC requests on WiFi networks.
• 10% of G1 requests were made on WiFi.
• Only a subset of the popular BlackBerry Pearl and Curve models are WiFi capable, with 16% of total RIM requests coming from WiFi enabled devices and 1% of requests were on WiFi.

In the US data, a striking "42% of iPhone requests are made on WiFi." That compares with an even higher 56% of iPhone requests in the UK.

WiFi Networks are considerably faster than carrier 3G networks and so the experience is generally better. I would imagine that as the "WiFi infrastructure" continues to develop we'll see higher numbers than this, with carrier networks being secondary to WiFi in terms of user preference. 

As WiMax/LTE/4G is built out we may see more and more "non-phone" devices accessing the Internet as well.