Set the distance on the phone, and then if you happen to leave the phone on the table where you just ate, and walk away, once the distance hits 5 feet or so the phone starts chirping or screaming..

You would not believe how many cell phones I see misplaced like this, and people do not even know they forgot the phone, or where they did for that matter.

For a couple more cents, and possibly even in conjunction with the highly annoying ringtone market, the manufactureres could make the warning customizable.

Me: Finishes the opera and begins to file down aisle exiting crowded concert hall.

Phone (talking in sexy voice after 5 feet is passed): Hey you sexy, do you want me? I want you! Come get me you devil. Don't leave me hanging...

Me: Continues to walk away, unable hear the phone.

Phone: (7 feet away now, and getting louder): Hey you absent minded deaf bastard at number (423) 745-3453 get your ass over here and collect your phone.

Then if I am obviusly too distracted to get my phone this little screaming feature might come in really handy.