If you are detected, the tickets will be cancelled. If miles are put into your account, you may have your account closed and lose all your miles and status.
It's not illegal, just against program rules. A good number of FTers have gotten severe sanctions for doing these things.
Not worth it, IMO.
Here's a long thread with horror stories.
http://www.flyertalk.com/forum/ameri...solidated.html
Safe travels,
Doc