1. File a missing item report on Delta.com
2. File a local police report, in the district where you landed

Here's the great thing about using "connected" devices like an iPad... generally, the person who takes it from the plane (usually a worker) isn't that bright and actually registers the unit themselves. Once this occurs and you have a police report filed, Apple will provide the user data to the police. Assuming you have the missing item report filed, Delta will cross the two with their local and contract employee base. If there's a match, device often returned, or at a minimum useless DL employee fired.

This situation recently happened to me, except a Kindle. When I called Amazon to "deactivate" the device, they indicated that it was already deactivated from my account and registered to another user. They then deactivated the new user at my request (mistake on my part). I filed the Delta missing item report as well as the police report. Delta and the police followed up but by that time the missing Kindle was no where to be found. However, I was notified that the member of the SFO ground team who claimed my Kindle as their own was terminated over the incident since Amazon had a record of the individual registering the device. For me, this was satisfaction enough! PS... San Francisco has a law about the "misappropriation" of lost property so this is how they were able to get a criminal charge against the person.

Good luck with your iPad!