There is a 50K bonus mile offer posted in the AmEx credit card forum. Warning - you only get the total bonus miles if you have never received AmEx bonus miles before. And be sure to print EVERYTHING related to the bonus - the offer itself, your registration page, etc. AmEx is noted for not honoring bonus mile offers.
Right now the DL offers are not great - I just earned 48K US Bank Flexpoints (worth $960) by charging $8K in airline tickets on two separate US Bank cards after all the bonuses. And since for a month I "double dip" with Worldperks, I also earn 8K WP miles (say they are worth $100). So my "return" is over 13% ($1,060/$8,000 spend). Can't get much better than that!
John