For people that are using Google Analytics, the good news is, this information is already tracked in Analytics, and the resolution is registered under the mobile -> devices section.
Unsurprisingly, most of my visitors come from an iPad, followed by the iPhone, then BlackBerry and then Samsung Android phones (I was surprised about Blackberry myself, but the demographics support it - Latin America).
On your Google Analytics
- On the left go to Audience -> Mobile -> Devices
- On the Primary Dimension (it shows Device Info, Branding, Service Provider, etc) select other then Screen Resolution