And if I recall correctly even Donald Trump was leading in the polls a time or two as well. And don't forget that polls in Iowa in February and March of 2011 were showing Huckabee in the lead.
It's a long campaign season and anything can happen. Walker is leading now but he'd better not get too comfortable or he's going to find himself overtaken. And you can't forget the advantage Bush will have in terms of money and establishment support.
Good catch, you are right now as I recall. So there were even more lead changes than I remembered. (this is the problem of posting before both cups of coffee have taken effect...:)