Scenario 1: Walker is telling the truth; which means he's so untrustworthy that he shared his private conversation with Cameron, and was dumb enough to think Cameron wouldn't deny it.
Scenario 2: Walker is a liar.
Scott Walker may or may not be a liar (I guess this incident shows that he probably is), but the bigger problem is that he's a backstabber. He sought out private sector unions' support (and won at least one major private union endorsement), then once re-elected, he signed a right-to-work bill.
As one of the rare Republicans that supports organized labor (though I hate that unions support Democrats), I have no room for such backstabbers in my party.
I supported Scott Walker at the beginning of this campaign, but I just don't think I'll ever be able to bring myself to trust him, and if we're going to have a president I don't trust, it might as well be a Democrat.