The py-usb 0.4.2 package does install on OS X. But you'll need to install libusb first. If you are using Fink then all's well. If you're using MacPorts you'll need to modify the setup.py Install libusb from MacPorts

whale:pyusb-0.4.2 rossetti$ sudo port install libusb

Patch the setup.py file for pyusb-0.4.2

37a38
> '-L/opt/local/lib',
39c40
<     extra_compile_args = ['-I/sw/include','-I/usr/local/lib']
---
>     extra_compile_args = ['-I/sw/include','-I/usr/local/lib','-I/opt/local/lib']

After that pyusb should install well.

whale:pyusb-0.4.2 rossetti$ sudo python setup.py install