As the last Tier 1 carrier to move forward with a 3G, high-speed data network, it makes sense that in step with those efforts T-Mobile USA is focusing on improving the mobile data experience, so replacing T-Zones with web2go, and adding Yahoo! OneSearch capabilities. See the T-Mobile press release for more details. What is less clear from the release is how the new pricing plans will impact existing voice and data customers, and/or how it will be bundled with Wi-Fi.
T-Mobile has always been the value play among the Tier 1 U.S. carriers, so any increases in data pricing need to be in step with improvements made to the experience (devices, network, services).