They had a great special on this on cable one time. Turns out that alot of them do the "marry you, get over here, and after getting citizenship, they accuse you of beating them" scam, and then you go to jail and they get it all.
I'm sure that some of them are quite sincere, but separating the wheat from the chaff would be a bitch.
ETA: I knew a guy at a job i had, he did this Mail Order Bride deal.
He was a pretty goofy looking guy, kinda goofy/scary looking, if you know what I mean. Anyway, he married some asian chick, and she wasnt' too bad looking from when I saw her at a company party.
One day he's in the training room on the phone, and he starts yelling "NO, NO, YOU CAN'T GO BACK, THE AIRPORT IS CLOSED, THE EMBASSY IS CLOSED" and then slams the phone down and knocks a table over. Nearby coworkers freaked out, some were heading for the exits... LOL
A week or so later he's walking down the hallway, limping.
A coworker asked, "Hey, what's wrong with your leg?"
He says, "Oh, Mai Lin stabbed me in the leg last night while I was sleeping..."
WTF!!!!