Question
Specifically AM EX and United Visa?
Thanks!
Answer
Welcome to FlyerTalk. There was a recent thread on this topic:
http://www.flyertalk.com/forum/showthread.php?t=521650&highlight=transfer
A search on "transfer" or "balance transfer" may yield other results.
Answer
Just a few days ago I called United Visa to transfer a balance from my AmEx - not a 1:1 mileage bonus, but I asked, and the agent gave me 1k miles.