I'm fairly sure that only Apple could add this feature. Almost everything having to do with the cellphone part of an iPhone is off-limits to third-party developers.
The iPhone actually does notify you (though it's only audio and not visual) if you have a missed call, voice-mail or text. It beeps every 10 (or is it 15) minutes. I would definitely prefer a visual notification. For that matter, how about visual notification for e-mails, tweets, etc?
[+] [-] wallflower|17 years ago|reply
http://iphone-rejectdb.appspot.com/detail.html?id=23
[+] [-] allenbrunson|17 years ago|reply
[+] [-] nshah|17 years ago|reply
[+] [-] pclark|17 years ago|reply
It lets you specify a "vibrate every x for y" setting - eg vibrate every 10 minutes for every missed call