My girlfriend was trying to sign up for the free Nintendo DS last year with the triple play. She had cable service at the time and was told it was for "new" subscribers to all three. She said that was ridiculous since she had only one of the services. She ended up talking to a supervisor who informed her they had very limited quantities of the DS. After much discussion, the supervisor said they couldn't give her the DS but they would credit her account for the value of the DS. It was a good trade-off since I had already gotten her a DS but bad for me since she said she was going to give it to me. Oh, well.
Yes, I have been told that if you threaten to leave any/all their services all together or ask for a supervisor you can negotiate to get the deals and promotions that are available to new customers. (I have friends and neighbors who have called and recieved free movie channel promos, giftcards . . .etc.)
So for existing customers it is highly YMMV, but you at least have a shot.
I am thinking about it, but with AT&T UVERSE for less in surrounding neighborhoods I am hoping it will be in mine soon. However, if I didn't have a Wii already this would be really tempting.