Yes, yes, Visual Voicemail is one of the major features.
The main point I was trying to make is that it's much easier to test an equation with few variables as possible. While Apple may be enjoying monthly revenue from AT&T customers I also believe that it was not the only factor leading them to want to stay with one carrier. Troubleshooting a brand new technology is much easier when you have fewer variables.
-Phillystax
My point in quoting your post was that it's the ONLY feature
isn't married to one business model then why is AT&T the only GSM carrier in the United States to offer the iPhone? And why is it only offered by one GSM carrier in other markets? I call shenanigans!