Is libusb the preferred method to talk to a device over USB (using AT commands) on OS X these days?
Is there any high level equivalent in cocoa?
On OS X, drivers are written using the IOKit and a subset of C++. More specifically, for USB devices, you probably want to start with Apple's USB Device Interface Guide and browse the USB parts of Hardware and Drivers section of the Mac OS X Core Library documentation set in XCode. (Edit: The latter resource is also available online.)