I don't know about BF:BC, but you should bear in mind that those 48hr trial codes have some limitations. I discovered that I couldn't play online split-screen COD 3 with a friend playing as a guest. We knew we had done this before when I had a proper account, so decided to use the one-month proper gold code I received when my 360 was repaired (for the second time) and we were able to play just like before.
There was also a problem trying to send a friend a picture from Halo 3. I believe that time I was informed it was something to do with not having a proper account.