It's being challenged on (state) constitutional grounds as we speak, actually. That'll be one ugly court case, but California's judiciary leaning the sane way on issues like this is part of why people wanted the proposition in the first place, so I'm cautiously optimistic.

On a more, ah, fundamental level, there's apparently a move getting started to get the IRS to revoke the Mormons' tax-exempt status because of their open involvement in the Prop 8 campaign.