Two things I'd love to see in my next mobile phone

1. When the phone's first switched on, it's locked. When the phone is locked, it display's the owner's name and number and - here's the twist - it can call certain predefined numbers. So if I drop it in the street and you pick it up, you can use it to call my house and tell me you've found it - but you can't use it for anything else.
2. I want to be able to call my own phone (e.g. from a landline), enter a PIN, and switch it to Loud Ringing remotely - so next time I lose it somewhere around my house whilst it's on silent, I can reactivate the ringer from another phone.

Both are basic software features you can probably implement in any modern handset - they don't use any new hardware or infrastructure. I guess since they make it easier to find missing phones, they make it less likely you'll end up buying another one, though...