This is what happened to myself and My sixteen yr old daughter :-(
I have iPhone 4 (white) and I for her iPhone 4 in black( I may take hers and upgrade it to 4S undecided just got it )
Anyways all my contacts went to HER phone as well and she deleted them ugh so guess what?!?! I lost ALL 220 contacts it took me forever lol
I'm not gonna freak cause it needed cleaning but still....
I should've known!