I left my Macbook on DL 31 this past Sunday, and realized my mistake as soon as I reached Manhattan. The Diamond help line wasn't terribly helpful -- she said she couldn't contact or connect me with anyone on the ground at JFK, and suggested in fill in the online form and wait a week -- but she did say I might improve my odds if I went straight back to JFK and spoke to a Red Coat. So I did; the Red Coat told me to go to baggage services, and a very helpful agent in the baggage services (domestic arrivals) tracked it down to an agent in the baggage services (international arrivals) desk (where obviously I should have gone in the first place), who coincidentally had sent me an email to the effect of "we think we've found your laptop" 20 minutes before, but which I had not seen.

Morals of the story: contact baggage services at the airport as soon as possible, hopefully while your airplane is still at its arrival gate. Show up in person if you can. And it helps to be a Diamond.

Good luck!