Here's some PTT info:
Cingular will most likely be offering two phones with ptt initially, the LG that cris has mentioned as well as the Samsung (not that far away),

Cingular's PTT can't compare to nextel's DC with initalialization of a two-way (cingular's is about 7seconds vs Nextel's ~2 seconds) but once initalized the two-way communication is imediate just like nextel's.

With Cingular's PTT you can convert a PTT conversation directly into a voice call, where as nextel you'd have to stop the DC, and look for the number and dial; it'll go straight into the voice call.

Another plus about cingular's PTT is the ability for group PTT's with any other PTT customer. Nextel you had to actually be part of the "group"

With nextel to DC with someone you had to get the DC number/pin and store that along with the users number, they eliminated the need for that with cingular, you just need the user's cell phone number to ptt.