How much are you willing to bet there will be hardfork? I'd be willing to bet some against a hardfork happening.

Would creating a contract for that bet be an interesting programming challenge? What would happen to that contract in a hard fork?

AFAIK hardforks are hard to detect by a contract, the contracts don't know anything outside the chain, if you split the chain, you can't tell there is another.

Of course I've briefly entertained the idea but on the other hand, a bet between two mostly-honest individuals doesn't need a blockchain contract, similar to how I don't need a blockchain contract to borrow money from my sister and pay it back later.

In any case, if there would be a hardfork, it would include the replay protection, so a simple but working solution would be to prepare a transaction for the fork chain and one for the non-fork chain, the participants can then publish the transactions once either chain becomes available and claim the reward.

It wouldn't be that hard in this case. You just have to watch for an address holding stuck ethers because of the bug, if the ethers move, it is a proof that a fork has happened.

It would be a good introduction to solidity and the 3rd party problem of smart contracts.

An example betting contract for anyone interested (note it was eventually refunded and not run):

https://etherscan.io/address/0xf4b8ccc5734ed7d2d8beb5fddd223...

And an overview with some reasonable criticism:

https://www.reddit.com/r/ethdev/comments/6sd8hl/mayweather_v...