Delta Skymiles is offering a 20% bonus on points transferred from American Express Membership Rewards through September 30. (Registration required.)

The bonus, though, isn’t expected right away:

Bonus miles will be posted 4 to 6 weeks following the end of the promotion.

The offer is not a good reason to transfer American Express points into Delta Skymiles. But if you’re going to make such a transfer anyway (eg to top off towards an award you know is available) then you might as well get the bonus…