Unfortunately it's not true. The 50% number comes from the latest AdMob Mobile Metrics Report (PDF). Nowhere in that report does it say that the iPhone accounts for 50% of all U.S. mobile web traffic. The 50% number refers to the Apple device's share of ad requests from smartphones. Smartphones as a group make up 33% of total U.S. ad requests so the iPhone's true share of US mobile web traffic is about 17%. Actually it's somewhat less than that as Ad Mob's iPhone numbers include requests from iPhone applications as well as browsing. iPhone apps are a very impressive and game changing phenomenon but are irrelevant to mobile browsing market share.
Worldwide the iPhone accounted for about 33% of smartphone ad requests. But again smartphones were responsible for about 33% of total mobile web traffic so the iPhone's share was about 11%. If we add traffic from the iPod Touch to the mix, which seems reasonable as it has the same browser as the iPhone, Apple's total share of AdMob requests rises to 17.9% worldwide and 27.1% in the US. Again, these figures are inflated by requests from in-app advertising.
While the 50% figure isn't really true, the Phone is a watershed device that has turned the industry upside down with it's intuitive, finger friendly touch UI and PC like a browsing experience that most users find as useful and compelling as the web on a PC.
So web designers absolutely need to insure that their sites deliver a great experience on the iPhone and Touch, but they shouldn't neglect the other devices that still drive over 82% of mobile web traffic either.