There's a BMG thread on the DVD Talk Forum. I discovered it a while back and bookmarked it. People post the codes there.
Where the codes originate from, I have no idea! But I'm not going to turn them down either. At this point BMG wouldn't be issuing them if they had problems with so many folks using them, I think.
PS - there's a thread about free magazine subscriptions on there too. Sometimes find some decent things. I've ended up with Paste, Blender, TV Guide (not sure why I got this, as I really don't watch much tv at all other than sportsl & news), and Motor Trend.