With an AMEX $49 annual fee I would suspect the Skypoints card to be competitive with the other airline cards with annual fees in $50 - $85 range, the mile earning would be one dollar would equal one skymile. It does appear the signup bonus of 2,500 miles is sure on the low side.
However, the next step for AMEX would be a free skypoint card with 1 skypoint earned for every $2. This would be competitive to the AA Bronze Citicard with no annual fee and a $2 to 1 AA mile ratio.
All in all it looks like AMEX is trying to recoup more of its "investment" in DL.