And how do you think it accesses this supposed "Gtalk" channel without using data? Unless it's doing so through SMS, the data channel has to be open for Gtalk to receive the "ping."
There are only a few ways information can get into your phone: the voice channel (which includes SMS), the 1xRTT radio, the EVDO radio, the 4G radio, WiFi and Bluetooth. For purposes of discussion, I think we can exlude BT, and I acknowledge that if WiFi is on you'll get data regardless of the "always on data" setting.
That leave the voice channel or the phone data channel. I guess it's possible google is using the SMS channel to ping the app, but I doubt it. Sprint would not be happy about that.
If data is off, the phone is simply unaware of any information trying to reach the phone on the data channels. For any non-voice information to reach your phone, data's got to be on. Now it's certainly possible the phone could turn data on every minute, let Gtalk check for input, then turn off again. I don't know, but I think it's unlikely.
You can have GTalk on and Always on Data off, and get push. Or you can have Always on Data on, and GTalk off, and get push. Turn both off, no push. This implies to me that either option turns data on and keeps it on.