Data points and details on gift options:
I cleared the MM threshold on a flight back to ATL on Memorial Day. Pretty much as soon as I saw credit for that flight post to my account, a new little "Million Miler" legend showed up alongside my Medallion level and Amex membership when I log in to DL.com. About two days after Memorial Day, I received an email welcoming me to MM status and inviting me to select a gift on the website. About three weeks after Memorial Day, I received a card in the mail that was basically the same as the email, and on the same day I received a new Medallion package with new bag tags bearing the MM logo.
When I went through the steps to see the gift selections, the options were from Hartman and Tiffany, as reported upthread. The Hartman selections included about a half dozen suitcases and briefcases. The Tiffany seletions included maybe as many as a dozen options, including a silver pen, a silver business card holder, a silver link men's bracelet, an Elsa Peretti bean or teardrop necklace (forget which), plain oval cufflinks, and airplane cufflinks.