Any unlocked (or Cingular-branded) 8700, 8800, 7130c/7130g, or 8100 running OS 4.2 (here's the key) and provisioned for PTT service (the other key) should have the functionality to utilize PTT on the handset. There is a service book associated to the service; when you open the PTT application, it will connect back to the carrier, check provisioning, and, if you are correctly provisioned, will download the service books with the gateway information to your handset and allow you to move on into the application and functionality.

This has been confirmed as working, just so you know.