To target specifically to mobile browsers and devices, simply log into your account and access the campaign you want to change. Within that campaigns settings, look for the “Networks, devices and extensions” and select “edit”

You should see this:

Next, select “Let me Choose” and un-check the “Desktop and Laptop computers” while keeping the “mobile” radio button selected.

The latest upgrade Google has made allows advertisers to target to specific mobile carriers and mobile platforms (iphone, Andriod, PalmOS etc). This can be useful for advertisers who have applications that are specific to the iphone mobile platform and need to target only to this platform.

The official Google annoucement can be read here