Quote from AEONN. on the 9th of July 2013:
"Will I keep my ranked standing (tier and division) when I transfer to Oceania?
Sort of. League standings are relative to the other players on your server, so when you transfer to Oceania you'll be placed based on how you stack up against the other ranked players who make the switch. In most cases, you'll find yourself in a higher tier and division than your original server.
You'll still have one placement match before you're officially placed into a league, but your prior ranked standing will be the key determiner. If you think some nefarious ghosts in the machine got ahold of your ranking at some point in the transfer process, please contact player support."
I call bullshit on this on riot.