They're currently split, ever so slightly, with the 6s. The AT&T model supports a couple extra bands vs. the Verizon.
It's not just the bands. The 6 is capable of VoLTE, but VoLTE isn't available everywhere on every carrier. So it still has to be able to drop down to 1x, which means CDMA for Verizon, Sprint, and US Cellular, while AT&T and T-Mobile use GSM.
You'd think Apple could put out just one model when the 1x networks go away.
Really? I thought there were only two versions in the US: Sprint and Everything Else
Per http://www.apple.com/iphone-6s/specs/ under "Cellular and Wireless", there are two models of the 6s and the 6s Plus, with the only difference being the presence of LTE Band 30. My understanding was that Band 30 (in the US) is exclusive to AT&T, but I don't know that.
That hasn't been true for a while. IIRC The Sprint model of the 6S is a better world phone because it has more bands (including Chinese bands); the current ATT model swaps some of those for the ATT 700mhz bands.
I believe it was true on the 5S but I haven't looked since.
Not true on the 6S. The AT&T model covers more LTE bands (addition of band 30) and has otherwise identical frequency and network support.
I believe they've always been split, as I recall there are 4 different iPhone 6 models supporting different frequencies / protocols.
Verizon supports GSM now?
At some point all voice calls will go over LTE and Verizon's CDMA voice network will be retired, but they're not going to adopt another 1x technology.