This came with July's Delta SkyMiles Card:
Earn Triple SkyMiles now through September 30 when you use your Delta SkyMiles Card to purchase Be My Guest Cards from American Express.
http://amex.etoll.net/amexbemyguest/

The small print says:
" The $5.95 fee per card includes all personalization, order processing, shipping, and 24 hour toll free Customer Service."

The highest denomination is $300, so the cost per mile is 0.66.
One can also "reload" the card for $2.95, but I'm not sure if it will qualify for triple miles. It looks like they will offer volume discounts, at least they mention 25 or 45 cards.

P.S.
This post belongs here, but it belongs as well to regular Buzz, Delta and Amex forums.
I'll post it there too, but life on FlyerTalk becomes terrible complicated!