It's available for download only to members of Apple's Developer Connection (ADC). Apple claims the SDK provides the necessary files to begin Carbon development. CarbonLib 1.4a supports Mac OS 8.6 or greater. Apple plans for the release to accelerate the shift of Classic applications to OS X.
ADC membership is available in Select and Premium levels, costing $500 and $3500 respectively. Each level of membership offers different levels of access to Apple's developer materials and support.