Mobile ad network AdMob has released its Mobile Metrics Report for April 2009. The report compares Smartphone market share to mobile web usage, and finds that Smartphones accounted for almost three times more usage than their relative market share in April. The report also compares usage of mobile websites to usage of HTML sites on mobile devices, finding that relative usage of both is highest on Apple and Android devices. 
The emergence of devices such as the iPhone and HTC Dream (G1) that deliver more compelling Internet experiences has generated discussion of which web consumers will use on mobile mobile websites or HTML sites. The report compares requests in AdMobs network to Net Applications.com Global Market Share Statistics for April 2009 to determine that iPhone and Android users both over-index in their share of usage on both mobile web and HTML  websites.
While Gartner estimated that global Smartphone sales represented 12% of total device sales in 2008, 35% of AdMobs worldwide ad requests in April 2009 came from Smartphones. The iPhone OS had 8% of the Smartphone market, but generated 43% of mobile web requests and 65% of HTML usage.
The Android OS share of the Smartphone market was less than 1%, but generated 3% of mobile web requests and 9% of HTML usage. The Symbian OS had 52% of the Smartphone market, but
generated only 36% of mobile web usage and 7% of HTML usage. Usage of mobile websites greatly outpaces usage of HTML sites on Smartphones running the Symbian and RIM Operating Systems. 24% of US requests were made over a wi-fi network. The top five wi-fi devices in terms of usage were the iPhone, iPod touch, Sony PSP, HTC Dream (G1), and the HTC Dash.
The comparison of Smartphone market share to mobile web usage in the report is based on Gartner's 'Handset Sales by Operating System' research from 2008, and ad requests in AdMobs network of mobile websites and applications in April 2009. The mobile web to HTML web comparison uses data from Net Applications.com Global Market Share Statistics April 2009 to show HTML usage
You can access the full report here.