How about this - I win if before he leaves office I can point to a speech Obama gave in which he advocates means testing Social Security. Otherwise you win. The speech has to be given after today, so you don't fear this is some kind of trick.
If I win I get $100 from you. If you win I give you $233. But with these odds I'm indifferent to making the bet. So for me to be willing to bet I want you to agree that if Obama makes such a speech you have to pay me right away.
That works for me, with one little change. The end of his term needs to be counted as the end of a presidential election he doesn't win, rather than the inauguration of his successor. This is because the reason I don't think its very likely is that the political effects on him would be dire, so if he does it as a lame duck president he has nothing to lose. I'm still willing to take the risk on his second term since even a second-term president is subject to some political forces.
And as a clarification, I take "means testing" to mean increasing...
I would like to propose this as a thread for people to write in their predictions for the next year and the next decade, when practical with probabilities attached. I'll probably make some in the comments.