A new service by comScore, Device Essentials™, provides ".. reporting on digital traffic by device, which includes computers and other devices, defined as mobile phones, tablets, music players, e-readers, gaming devices, and other web-enabled devices".
See "comScore Introduces Device Essentials™ for Measuring Digital Traffic from All Devices, Enabling Optimization of Marketing Strategies and Customer Experience" - here.
One of comScore's recent findings (for the US) shows that Apple's mobile devices (iPhone, iPad) use Wi-Fi services to access the internet, while Android owners (phones and tablets) are using 3G services. The difference is huge, as the table shows.
It seems that AT&T moves (until recently the only 3G provider for Apple devices) - implement usage-based billing (certainly with the problems reported here), deploy free Wi-Fi services (despite what AT&T executive said here) and charge $20/month for tethering (here) were very effective.
Or maybe it relates to the signaling problem? (see "NSN: Android and Blackberry Phones Overload Networks with Signaling" - here).
For iPads, it may also depend on the share of 3G devices vs. Wi-Fi only devices. AppleInsider estimated on February that 38% of all iPads (globally) were Wi-Fi only.
See "Over 60% of Apple's first-wave iPad 2 production to be 3G models" - here.