Cingular today launched push-to-talk service. Both Sprint Nextel and Verizon Wireless already have had push-to-talk features for quite some time (it originated with Nextel). Unlimited use of PTT on Cingular costs $9.99 per month or double that for family plans.
The Cingular implementation has a few cool twists, including being able to transfer a PTT conversation ito a regular call. There is also a presense feature so you can know someone is available before starting a conversation.
Phone compatibility is currently slim, with just the freshly announced LG F7200 able to work with PTT. Cingular noted that the Samsung D357 should launch soon and will also be PTT savvy.