After only one month the Motorola Droid already has 25% of the Android handset traffic in the US

Click the images to enlarge

Source: Page 5 of the AdMob Mobile Metrics report, November 2009
Note: AdMob is a mobile ad server, which means that they provide the mechanism that puts ads onto mobile sites. This means that their traffic figures - which handsets they deliver ads to - provide a good proxy for mobile internet traffic.