I personally have always wondered why some cell phones did not have a little proximity switch much like an RFID thing. The proximity tag (RFID) could be put in ones wallet or handbag, or on ones personal effects.

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.