Anyhow, back when we were getting our AT&T appointment set up, I mentioned how it was nice because now I could call Comcast and schedule our turn off date. The AT&T rep told me that I shouldn't do that yet because they "wouldn't want any interruptions of service" and AT&T's email confirming my appointment told me the same thing. I ended up not calling Comcast anyhow just because I knew our appointment time may have needed to change.
So Saturday I called Comcast to get service shut off. They took me through the usual stuff ("We're sorry to lose you as a customer" etc) and then got to the part that I knew was coming: they offered me deals (in fact they offered me the triple play for $10 below list price). I turned them down and was happy with the service rep who did her job without being a jerk about it.
Anyhow, I realized that that's the reason why AT&T didn't want me to call and cancel service: if I called before I had AT&T service, Comcast would have the opportunity to try and get me to reconsider and could point out that AT&T has a long installation. So in "making sure I have uninterrupted service" AT&T is also making sure that I don't get to talk to Comcast until Comcast has already lost me. Pretty smart.