So, I've been doing some looking around, and I think a good way to encourage registrations would be to include other ways of registering and logging in.
I'm talking about Twitter and Facebook, a good percentage of my traffic comes from Twitter and though I haven't been working on the Facebook part lately, I will be adding a Like or Recommend button soon to the other buttons we have.
So, my plan is, when I redo the guest welcome box I'll be adding a column there with a nice list of links or buttons for logging in. I've uncovered some threads in the MyBB community forums that cover setting up logins with Facebook and Twitter, so it should be doable with some effort.
I don't have an ETA, but it's pretty high on my list let me tell you ;)