Couple things you may try
Contact Apple with your iPad Serial #. Ask them if THEY can track it...and/or wipe it, render it useless if it's plugged in to iTunes...or if it seeks an update/restore/et. al (Not sure if this is a possibility...seems as it should be, since everything is electronically tracked thru iTunes!)
Contact Delta. Find out where that airplane went to next (and possibly even the next stop). You can track this online if you know the following flight number. Then, run an ad on Craigslist under Lost and Found (with Reward as someone else mentioned) in those particular markets. You may even contact those market newspapers and run a small price L&F ad in their classified section.
You may even mention in the ads you're looking to have it returned, first. BUT, being a sophisticated electronic device...it CAN be tracked and you WILL contact the local authorities...as an over $500 theft in MOST states constitutes Grand larceny/theft, and is construed as a felony.
You may take that step anyway...if Delta will give you the flight information. Sounds like you made a claim with them, but I would also let them know (and write to management) that you are contacting authorities as there is personal information on it....and you're concerned the cleaning crew (either employees OR contractors of Delta) would have been the first to notice the missing iPad.
Check with your homeowner's/renter's insurance. (Already mentioned). Did you purchase your flight on American Express? Often they offer Travel insurance. Possible with other credit cards...you may want to check.
Again...I would try Apple right off the bat...see if there's a way they can track it, or put some type of "APB" or tag on the serial number when it tries to connect to their servers. I would also contact Delta...work your way up the chain and see what can be done.
Thanks to the previous poster...12,000 laptops a week is extreme. Possibly, nobody will care...as they hear it all the time...but maybe, you'll get lucky and find a persistent employee that'll help you out.
Don't listen to the "I can't believe you left it in the seat" crowd. What a joke! EVERYONE has made a mental error...human nature. Those that say they don't, won't, or never will are lying to themselves! We all do it! At least 12,000 a week
Good Luck bro...I wish you the best. Being stolen from is certainly a violation and it hurts. (I had a thousand bucks stolen from my wife's purse at a "popular ice cream shop" last summer...and came to find out it was the assistant manager of the store! Unreal what folks will do these days...)
I wish you the best...That ***** is tough to get over!~
J