WARNING!
I received a similar offer in December and switched from MCI. Terms were (a) ATT pays switching fee, (b) no basic fee for calling plan for 12 months and (c) $20 credit per month in each of first four months, $40 credit if bill above $100.
When I received my first bill no credit whatsoever appeared. I called ATT yesterday, and they did not have any entry in my file and refused to honor the offer (only agreed to carry switching fee). I then talked to a supervisor who had never heard of such an offer. When I very politely clarified that without any benefits I would naturally switch back to MCI, her comment only was "If this is what you want to do, then go ahead."
These are highly questionable sales practices, as clearly the promised benefit is part of my contract with ATT. I wonder what they might say if I would -- in turn --unilaterally declare paying my bill as not being part of the contract. I informed the Better Business Bureau on this case.