Poster HansDKNY on FlyerTalk forums has mentioned to UA Fan (who runs the Delta AmEx Master Thread) that you might get an offer for 50,000 bonus miles when signing up for SkyMiles, but it may be targeted. Have you tried this and been successful?
The generic public offer for a Delta AmEx is a measly 10,000 miles, but there's other ways to get a better offer if signing up for SkyMiles doesn't work, including this nifty trick:
Browse this Delta AmEx link incognito for best results. Enter your SkyMiles number and last name. You could, like some FlyerTalkers, be offered 35,000 when spending $750 in 3 months.
Let us know your success stories here.